jQuery.animate() with css class only, without explicit styles
The jQueryUI provides a extension to animate
function that allows you to animate css class.
edit: Example here
There are also methods to add/remove/toggle class which you might also be interested in.
Weston Ruther built a similar thing using the WebKit proposal for css transitions:
http://weston.ruter.net/projects/jquery-css-transitions/
This implementation might be useful for you.