Same issue tonight September 30.
While watching Apple TV2, we abruptly had our movie interupted with the message that the "home-sharing" Apple computer was not "authorized".
However - we could view all our iTunes content on the actual computer (it was indeed "authorized") - it just wasn't recognized as such on Apple TV.
We've had no prior issues with our system and had no recent upgrades.
To trouble shoot I: 1) unplugged and plugged-in Apple TV, 2) deathorized and reauthorized computer, and 3) restored settings on Apple TV. All without any change. Have left a service message with Apple.
Perhaps a problem with the iCloud change over process? Very frustrating as currently unable to view TV or Movies on AppleTV.