My Wacom tablet, for some weird reason, sometimes just stop working. Usually I logOut/logIn but it's a drag. I just discovered that launching /Library -> StartupItems -> Tablet -> TabletDriver.app brings it to life again.
[robg adds: I have the same problem, usually after switching to another machine via my USB kebyoard/monitor switchbox. This solution definitely works for me.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030516160726530