Since Apple released iOS 5.1.1 for the Apple TV last week alongside iTunes 11, many users have reported problems updating to the new software over an ethernet connection. This appears to be the result of nasty bug in the latest Apple TV OS. Some Apple TVs even failed to boot back up after the update wouldn’t stick.
If you’ve been having problems updating to 5.1.1 on your Apple TV, then try updating over a Wi-Fi connection or manually restoring in iTunes.