The iTunesHelper application is used to automatically detect iPod's/iPad's and iPhones, when you connect these to your Mac. The application launches iTunes, and (if configured) synchronizes stuff. Very handy, but it can be quite annoying.
E.g. I have an iMac (main 'PC' around the house), and a MacBook. All the synchronization stuff is done on my iMac, and I don't sync anything on my MacBook. But when I attach my iPhone to my MacBook for Internet access (tethering), it launches iTunes and wants to synchronize with an empty iTunes. Something I definitelly don't want.
This feature can be turned off by removing the iTunesHelper application entry from the Login Items in your account settings, but turning it back on is something completely different. I accidentally removed it from my iMac, which wasn't supposed to happen....
iTunesHelper application in Login ItemsThe iTunesHelper application is hidden, so you don't see when you browse the hard disk with the Finder app. It is, in fact, part of the iTunes application (package). So in order to add it again to the login items you need to open the iTunes application package, and drag the application to the Login Items dialog in the System Preferences.
The quickest way to do this is to open a folder location from the Finder (⌘⇧G) and enter the following location:
[iTunes 10.3 and earlier]
/Applications/iTunes.app/Contents/Resources/[iTunes 10.4 and later]
/Applications/iTunes/Contents/MacOS/
This location holds the iTunesHelper.app.
iTunesHelper location
Drag the iTunesHelper App to the Login ItemsAfter dragging it to the Login Items everything should work as it did before (after you re-login to the affected account).