A number of users have reported crashes (unexpected quits) after updating to iTunes 10.1. The crashes can occur when users attempt to access movies, Ping or other components. The crashes may also occur as soon as iTunes 10.1 launches or during sync.
Reinstall Mac OS X Combo Updater. Download and reinstall either the Mac OS X 10.6.5 combo updater or Mac OS X 10.5.8 combo updater (depending on whether you are using Snow Leopard or Leopard, respectively). Doing so may overwrite files that were rendered corrupt or otherwise problematic after the update to iTunes 10.1.
Downgrade to iTunes 10. If reinstalling the Mac OS X combo updater and other fixes do not work, you can follow these instructions to downgrade iTunes 10.1 to iTunes 10.0.