If your iTunes refused to quit and relaunch itself everytime, this program maybe the culprit.

I went to Activity Monitor and found 2 instances of genredetection (part of the iWoW to auto detect the genre in the EQ). I force quit them and iTunes finally quit.

And then I discovered Shock J's comments ... "Latest version of iWow (2.0.2.255) has a process called "genredetection." If iTunes 7.5 (in Leopard) quits unexpectedly, the genredetection process doesn't quit and forces iTunes to automatically relaunch and spawns a second genredetection process. Here is where the problem starts. Now, whenever you try and quit iTunes normally, either via command key or file menu, the genredetection process continue to force iTunes to relaunch and continues to respawn new processes of itself. At this point, iTunes becomes practically un-quitable. You now have to go into the Activity Monitor application and force quit all the "genredetection" processes that have spawned and only then can you actually quit iTunes. I consider this a serious bug. Seeing as how we all know SRS's support is non-existent, we'll see if this ever gets fixed."