I was recently asked to help someone who was getting errors after iTunes syncs. The message was “An unknown error occurred. (1660)”
It didn’t happen after every sync, but if you exited iTunes, rebooted the iPhone and you let it sync, it happened every time.
The resolution was to uninstall something called RecoveryGuard. Recovery guard ostensibly prevents iTunes from putting your phone into recovery mode and convinces iTunes that your iPhone always has the latest software. Somewhere along the way, Apple released an update for iTunes that doesn’t like this. I can’t say for certain exactly when it showed up, but it is present with iTunes 10.4.1 on OS X Lion.