This happened to me as well. I was running 10.7 and one day 11 was installed without request or confirmation even though I had the "automatic update" checkbox unchecked.
To troubleshoot, I have backed up my music and itunes DBs, uninstalled itunes and the related programs (Bonjour, Apple Application Support, Apple Software Update, Apple Mobile Device Support), deleted all the files in my profile and in Program Files related to itunes and Apple, and rebooted. I reinstalled 10.7, unchecked auto update, and did not import anything back in so I essential had a blank/empty itunes. After about 48 hours, 11 had been installed again.
I feel like there must be an update setting that is forced to "on" even though I unchecked the option myself. Am I missing registry entries or some file that is controlling this?