I forgot about the QT component (I sort of inherited mine a long time ago).
I didn't think anything Apple could associate with any containers/files except for mov, mp4, flv and any other Apple-approved formats (so NOT avi, wmv, mpeg. etc.). I don't think you can play mpegs in itunes, so didn't think Apple TV would play them. You CAN play them (and everything else too) in Quicktime/Front Row, but need that component for that too as well as Perian.