Couple days ago, iTunes suddenly showed me this box instead of the usual sync indicator when I synced my iPhone to the iTunes:
iTunes cannot read the contents of the iPhone. Go to the Summary tab in iPhone preferences and click Restore to restore this iPhone to factory settings.
Restore iPhone to factory settings? Restoring it means loosing lots of data since the last backup. Heck no. I Googled this case a bit, and this tutorial saved me. It turned out that the iTunes index file on the iPhone was corrupted. It needed to be removed so the iTunes would build the new one and then synced with the iPhone.
Ah, how resourceful and wonderful the internet can be? It saved me. Again.