I have had this issue for a while but now I have an idea whats causing it I'll post it and see if theres any solutions.
Whenever I close iTunes there is about a 10 second delay before it re-opens. To begin with I had no idea what what causing this but then I installed last.fm and this kept popping up saying Preparing For iPod Scrobbling even when there was no iPod connected. So basically, I think that iTunes constantly assumes there is an iPod connected and so opens for syncing when there is nothing actually there.
Any idea why this would be??