After upgrading to Snow Leopard, I had issues with assigning Exposé and Dashboard to extra mouse buttons: I could assign them in the System Preferences panel, but some would not work.
I have found the solution in this Apple Discussions thread: you need to delete ~/Library » Preferences » com.apple.symbolichotkeys.plist, then log out and back in again.
This fixed the issue for myself, as well as others in the linked discussion.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090830031411310