If you are talking about CSS3 transitions/animations (CSS3Please.com demo here), be aware that IE is lacking in support for these almost entirely - it is unclear but unlikely that the new IE9 will have CSS3 transitions.
This is a good article written about CSS3 transitions/animations this week:
http://www.onextrapixel.com/2010/11/17/a-comprehensive-guide-to-css-3-transitions/.
Decide whether transitions are necessary for IE based on the demographics/browser preferences of your user base, and also knowing that IE users are far less likely to expect such transitions (because fewer developers make the effort). Unless it is truly critical that your site looks gorgeous in IE it s usually acceptable to make IE functional, while implementing transitions for the more advanced browsers.