Click on your icon at the top of the iChat buddy list and choose the Edit Picture option. The familiar edit mode window will appear, but with the addition of a new Effects button to the right of the Take Snapshot button. Press it and a nice little transparent menu of all available photo effects will appear.
Not mind blowing, but nice and I wasn't able to find another hint about it. Enjoy!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071119054059704