There have been a number of reports that people are having all kinds of issues with iTunes 7 (funny how this happens every time Apple updates iTunes. I would wager that iTunes is Apple's most used app, and I'm including OS X). Macworld UK has gathered up a number of links to Apple Knowledge base articles that lists bugs, and offer solutions.
If you're experiencing any trouble with iTunes 7 you should check this out.
I have updated iTunes on several Macs and have run into no problems, but your mileage may vary.