Use spotlight to drag everything related to iTunes to the trash, including the app. Then install iTunes 6 or whatever version was included on your mac dvd (Probably not iTunes 6, I guess) via pacifist.
I wouldn't recommend going backwards, though. The OS may still think that you've updated to iTunes 7, and future software updates may try to find iTunes 7 on your mac for updates to work.
If you can't find another solution to whatever bug you have, I'd just wait until they update iTunes again, which should be shortly.