A few days ago, I managed to get my AC3 codec to do AC3 passthrough on the AppleTV. It wasn’t that hard once I stopped making stupid mistakes. Now, if you have a video file with AC3 audio along with a dolby digital decoder, you can play it on your AppleTV and enjoy full 5.1 dolby digital surround.
David Conrad later sent me a patch to make it work on PPC based macs. It is annoying not having a decoder to test these things on. I suppose that just adds to the challenge.