CSS-Transforms are not possible to animate with jQuery, yet. You can do something like this:

function AnimateRotate(angle) {
    // caching the object for performance reasons
    var $elem = $('#MyDiv2');

    // we use a pseudo object for the animation
    // (starts from `0` to `angle`), you can name it as you want
    $({deg: 0}).animate({deg: angle}, {
        duration: 2000,
        step: function(now) {
            // in the step-callback (that is fired each step of the animation),
            // you can use the `now` paramter which contains the current
            // animation-position (`0` up to `angle`)
            $elem.css({
                transform: 'rotate(' + now + 'deg)'
            });
        }
    });
}

You can read more about the step-callback here: http://api.jquery.com/animate/#step

http://jsfiddle.net/UB2XR/23/

And, btw: you don't need to prefix css3 transforms with jQuery 1.7+

Update

You can wrap this in a jQuery-plugin to make your life a bit easier:

$.fn.animateRotate = function(angle, duration, easing, complete) {
  return this.each(function() {
    var $elem = $(this);

    $({deg: 0}).animate({deg: angle}, {
      duration: duration,
      easing: easing,
      step: function(now) {
        $elem.css({
           transform: 'rotate(' + now + 'deg)'
         });
      },
      complete: complete || $.noop
    });
  });
};

$('#MyDiv2').animateRotate(90);

http://jsbin.com/ofagog/2/edit

Update2

I optimized it a bit to make the order of easing, duration and complete insignificant.

$.fn.animateRotate = function(angle, duration, easing, complete) {
  var args = $.speed(duration, easing, complete);
  var step = args.step;
  return this.each(function(i, e) {
    args.complete = $.proxy(args.complete, e);
    args.step = function(now) {
      $.style(e, 'transform', 'rotate(' + now + 'deg)');
      if (step) return step.apply(e, arguments);
    };

    $({deg: 0}).animate({deg: angle}, args);
  });
};

Update 2.1

Thanks to matteo who noted an issue with the this-context in the complete-callback. If fixed it by binding the callback with jQuery.proxy on each node.

I've added the edition to the code before from Update 2.

Update 2.2

This is a possible modification if you want to do something like toggle the rotation back and forth. I simply added a start parameter to the function and replaced this line:

$({deg: start}).animate({deg: angle}, args);

If anyone knows how to make this more generic for all use cases, whether or not they want to set a start degree, please make the appropriate edit.

---

The Usage^{...is quite simple!}

Mainly you've two ways to reach the desired result. But at the first, let's take a look on the arguments:

jQuery.fn.animateRotate(angle, duration, easing, complete)

Except of "angle" are all of them optional and fallback to the default jQuery.fn.animate-properties:

duration: 400
easing: "swing"
complete: function () {}

1st

This way is the short one, but looks a bit unclear the more arguments we pass in.

$(node).animateRotate(90);
$(node).animateRotate(90, function () {});
$(node).animateRotate(90, 1337, 'linear', function () {});

2nd

I prefer to use objects if there are more than three arguments, so this syntax is my favorit:

$(node).animateRotate(90, {
  duration: 1337,
  easing: 'linear',
  complete: function () {},
  step: function () {}
});

Another answer, because jQuery.transit is not compatible with jQuery.easing. This solution comes as an jQuery extension. Is more generic, rotation is a specific case:

$.fn.extend({
    animateStep: function(options) {
        return this.each(function() {
            var elementOptions = $.extend({}, options, {step: options.step.bind($(this))});
            $({x: options.from}).animate({x: options.to}, elementOptions);
        });
    },
    rotate: function(value) {
        return this.css("transform", "rotate(" + value + "deg)");
    }
});

The usage is as simple as:

$(element).animateStep({from: 0, to: 90, step: $.fn.rotate});

Without plugin cross browser with setInterval:

                        function rotatePic() {
                            jQuery({deg: 0}).animate(
                               {deg: 360},  
                               {duration: 3000, easing : 'linear', 
                                 step: function(now, fx){
                                   jQuery("#id").css({
                                      '-moz-transform':'rotate('+now+'deg)',
                                      '-webkit-transform':'rotate('+now+'deg)',
                                      '-o-transform':'rotate('+now+'deg)',
                                      '-ms-transform':'rotate('+now+'deg)',
                                      'transform':'rotate('+now+'deg)'
                                  });
                              }
                            });
                        }

                        var sec = 3;
                        rotatePic();
                        var timerInterval = setInterval(function() {
                            rotatePic();
                            sec+=3;
                            if (sec > 30) {
                                clearInterval(timerInterval);
                            }
                        }, 3000);

this is my solution:

var matrixRegex = /(?:matrix\(|\s*,\s*)([-+]?[0-9]*\.?[0-9]+(?:[e][-+]?[0-9]+)?)/gi;

var getMatches = function(string, regex) {
    regex || (regex = matrixRegex);
    var matches = [];
    var match;
    while (match = regex.exec(string)) {
        matches.push(match[1]);
    }
    return matches;
};

$.cssHooks['rotation'] = {
    get: function(elem) {
        var $elem = $(elem);
        var matrix = getMatches($elem.css('transform'));
        if (matrix.length != 6) {
            return 0;
        }
        return Math.atan2(parseFloat(matrix[1]), parseFloat(matrix[0])) * (180/Math.PI);
    }, 
    set: function(elem, val){
        var $elem = $(elem);
        var deg = parseFloat(val);
        if (!isNaN(deg)) {
            $elem.css({ transform: 'rotate(' + deg + 'deg)' });
        }
    }
};
$.cssNumber.rotation = true;
$.fx.step.rotation = function(fx) {
    $.cssHooks.rotation.set(fx.elem, fx.now + fx.unit);
};

then you can use it in the default animate fkt:

//rotate to 90 deg cw
$('selector').animate({ rotation: 90 });

//rotate to -90 deg ccw
$('selector').animate({ rotation: -90 });

//rotate 90 deg cw from current rotation
$('selector').animate({ rotation: '+=90' });

//rotate 90 deg ccw from current rotation
$('selector').animate({ rotation: '-=90' });

jQuery transit will probably make your life easier if you are dealing with CSS3 animations through jQuery.

EDIT March 2014 (because my advice has constantly been up and down voted since I posted it)

Let me explain why I was initially hinting towards the plugin above:

Updating the DOM on each step (i.e. $.animate ) is not ideal in terms of performance. It works, but will most probably be slower than pure CSS3 transitions or CSS3 animations.

This is mainly because the browser gets a chance to think ahead if you indicate what the transition is going to look like from start to end.

To do so, you can for example create a CSS class for each state of the transition and only use jQuery to toggle the animation state.

This is generally quite neat as you can tweak you animations alongside the rest of your CSS instead of mixing it up with your business logic:

// initial state
.eye {
   -webkit-transform: rotate(45deg);
   -moz-transform: rotate(45deg);
   transform: rotate(45deg);
   // etc.

   // transition settings
   -webkit-transition: -webkit-transform 1s linear 0.2s;
   -moz-transition: -moz-transform 1s linear 0.2s;
   transition: transform 1s linear 0.2s;
   // etc.
}

// open state    
.eye.open {

   transform: rotate(90deg);
}

// Javascript
$('.eye').on('click', function () { $(this).addClass('open'); });

If any of the transform parameters is dynamic you can of course use the style attribute instead:

$('.eye').on('click', function () { 
    $(this).css({ 
        -webkit-transition: '-webkit-transform 1s ease-in',
        -moz-transition: '-moz-transform 1s ease-in',
        // ...

        // note that jQuery will vendor prefix the transform property automatically
        transform: 'rotate(' + (Math.random()*45+45).toFixed(3) + 'deg)'
    }); 
});

A lot more detailed information on CSS3 transitions on MDN.

HOWEVER There are a few other things to keep in mind and all this can get a bit tricky if you have complex animations, chaining etc. and jQuery Transit just does all the tricky bits under the hood:

$('.eye').transit({ rotate: '90deg'}); // easy huh ?

To do this cross browser including IE7+, you will need to expand the plugin with a transformation matrix. Since vendor prefix is done in jQuery from jquery-1.8+ I will leave that out for the transform property.

$.fn.animateRotate = function(endAngle, options, startAngle)
{
    return this.each(function()
    {
        var elem = $(this), rad, costheta, sintheta, matrixValues, noTransform = !('transform' in this.style || 'webkitTransform' in this.style || 'msTransform' in this.style || 'mozTransform' in this.style || 'oTransform' in this.style),
            anims = {}, animsEnd = {};
        if(typeof options !== 'object')
        {
            options = {};
        }
        else if(typeof options.extra === 'object')
        {
            anims = options.extra;
            animsEnd = options.extra;
        }
        anims.deg = startAngle;
        animsEnd.deg = endAngle;
        options.step = function(now, fx)
        {
            if(fx.prop === 'deg')
            {
                if(noTransform)
                {
                    rad = now * (Math.PI * 2 / 360);
                    costheta = Math.cos(rad);
                    sintheta = Math.sin(rad);
                    matrixValues = 'M11=' + costheta + ', M12=-'+ sintheta +', M21='+ sintheta +', M22='+ costheta;
                    $('body').append('Test ' + matrixValues + '<br />');
                    elem.css({
                        'filter': 'progid:DXImageTransform.Microsoft.Matrix(sizingMethod=\'auto expand\','+matrixValues+')',
                        '-ms-filter': 'progid:DXImageTransform.Microsoft.Matrix(sizingMethod=\'auto expand\','+matrixValues+')'
                    });
                }
                else
                {
                    elem.css({
                        //webkitTransform: 'rotate('+now+'deg)',
                        //mozTransform: 'rotate('+now+'deg)',
                        //msTransform: 'rotate('+now+'deg)',
                        //oTransform: 'rotate('+now+'deg)',
                        transform: 'rotate('+now+'deg)'
                    });
                }
            }
        };
        if(startAngle)
        {
            $(anims).animate(animsEnd, options);
        }
        else
        {
            elem.animate(animsEnd, options);
        }
    });
};

Note: The parameters options and startAngle are optional, if you only need to set startAngle use {} or null for options.

Example usage:

var obj = $(document.createElement('div'));
obj.on("click", function(){
    obj.stop().animateRotate(180, {
        duration: 250,
        complete: function()
        {
            obj.animateRotate(0, {
                duration: 250
            });
        }
    });
});
obj.text('Click me!');
obj.css({cursor: 'pointer', position: 'absolute'});
$('body').append(obj);

See also this jsfiddle for a demo.

Update: You can now also pass extra: {} in the options. This will make you able to execute other animations simultaneously. For example:

obj.animateRotate(90, {extra: {marginLeft: '100px', opacity: 0.5}});

This will rotate the element 90 degrees, and move it to the right with 100px and make it semi-transparent all at the same time during the animation.

Thanks yckart! Great contribution. I fleshed out your plugin a bit more. Added startAngle for full control and cross-browser css.

$.fn.animateRotate = function(startAngle, endAngle, duration, easing, complete){
    return this.each(function(){
        var elem = $(this);

        $({deg: startAngle}).animate({deg: endAngle}, {
            duration: duration,
            easing: easing,
            step: function(now){
                elem.css({
                  '-moz-transform':'rotate('+now+'deg)',
                  '-webkit-transform':'rotate('+now+'deg)',
                  '-o-transform':'rotate('+now+'deg)',
                  '-ms-transform':'rotate('+now+'deg)',
                  'transform':'rotate('+now+'deg)'
                });
            },
            complete: complete || $.noop
        });
    });
};