Under iTunes Preferences, Devices, be sure "Prevent iPods, iPhones, and iPads from syncing automatically" is checked.
Its probably best to step through all the preferences panes and make sure there's nothing nasty there.
I stopped updating at iTunes 11.-- because I think the App's been going steadily downhill since then, so there might be a setting I've not seen that could cause trouble. It''s definitely worth a look through prefs though.