If you own a G1 (first generation) Apple TV, you may have recently encountered an error message on your ATV unit when trying to watch content purchased on your computer, iPhone, iPod, or iPad.
The error, which reads "Your Apple TV is not authorized to play this content.", began appearing for some users when attempting to play movies or tv-shows on the older apple tv's that was purchased on another device.
While Apple has not recognized the error or posted a fix, there is a work-around to resolve the "not authorized" error message.
To resolve the error message, simply purchase any content directly from the Apple TV. (Tip: There are many "free" preview shows that would work great for this!)
Once the newly purchased content is downloaded and played, all of your other purchased tv shows and movies will now play properly.