Ok, by doing this hack, you can stream ANY video format sitting on your Mac to your AppleTV. I'm testing out an mkv file right now, and it works like a charm!
Download the unofficial Mac AirVideo client here:
http://dl.dropbox.com/u/6713110/MacAirVideoClient/Site/Download.html
Start playback of a video using live conversion from within this unofficial client, it should hand off the streaming video to Quicktime X.
Inspect the stream by viewing the Movie Inspector (check the "Window" menu for Quicktime). Write down that url.
Paste that url into Erica Sadun's Airflick app and boom now your AppleTV can basically play any video you throw at it! Looking forward to someone streamlining the process into a neat little app.
I'm considering reposting this in the TUAW comments in case anyone else finds it handy.
OPTIONAL
One extra little note: you don't have to necessarily download the unofficial Mac AirVideo client above. You can instead begin playback of a video from within the AirVideo iphone app, then go to the mac serving up the stream, and type "ps ax | grep ffmpeg", and grab the alphanumeric string following the --conversion-id flag.
Then paste into AirFlick the following:
http://[YOUR-SERVER'S-IP-ADDRESS]:45631/live-playback-2.4.0/index_[CONVERSION-ID].m3u8