Replying to [comment:3 scott.gonzalez]:
This works fine in master: http://jsfiddle.net/Z6mJU/110/
Unfortunately, that's not accurate. In that fiddle, JQueryUI isn't loaded. Loading JQueryUI reproduces the problem.
This modification of that fiddle better represents the problem, and also includes a workaround which itself demonstrates the problem: http://jsfiddle.net/Z6mJU/144/
With JQueryUI enabled, the first div--which has no class attribute in the HTML--doesn't animate, but jumps directly to the applied class.
The second div--which has a class attribute--does animate as intended.
By adding a dummy class to the third (and thus establishing a class attribute) before adding the animated addClass call, the animation applies as intended.