What can cause an iPhone 4 to not actually sync data when told to sync?
iTunes uses iTunesHelper.app to detect if an iPhone has been plugged in. This app is usually started as a login item. Therefore you could check if iTunesHelper.app ist listed here: Preferences -> Users -> Login Items. If it's not listed you can drag it back as described here: https://discussions.apple.com/thread/2313998?start=0&amp;tstart=0.