Apple has added Xvid to its online QuickTime Components list. The Xvid-for-QuickTime component, allows you to play and encode Xvid video. Apple states:
The Xvid component for QuickTime allows users to play & encode Xvids in QuickTime Player. Xvid is a video codec library following the MPEG-4 standard.
In theory, this means that the Apple TV could also support Xvid video playback.

Posted in Apple Software, Quicktime Player