Are there known DTS or Dolby Digital encoded videos readily available on Apple TV for testing?
After about a week of use, I have not come across any free content on AppleTV that is 5.1 encoded.  My search was by no means exhaustive, but even the previews for blockbuster movies seem to only be plain old stereo.

The good news is that various movies on Netflix (e.g. a Big Year) and iTunes (e.g. The Hunger Games) are Dolby Digital encoded, so if you play them on Apple TV, discrete audio channels are played back if your audio equipment is capable.

Edit:
Other notable Dolby Digital encoded content:  


iTunes festival 2012 - Usher (free, but probably not for long)