A solution to this problem is that any time you want to quit iTunes and not have it start back up, open up Activity Monitor and search for the process genredetection. Quit this, and iTunes should then behave properly and quietly quit when you want it to.
[robg adds: This tip appears (with more detail and screenshots) in this post in the author's blog. As I wasn't experiencing this problem, and didn't have a process named genredetection, I went digging for the cause, and found this thread on the Apple Discussions (which referenced a post from MacUpdate). It turns out that the genredetection process comes from the SRS iWOW plug-in, and it's supposed to quit if iTunes quits normally. It seems, at least in some cases, that's not the case. An alternative solution then is to disable the plug-in when you need to quit iTunes.]

