PLEX, media center for OSX and iOS now has a proof of concept client for Apple TV. This client, when installed on a jailbroken Apple TV, allow you to stream most any kind of video back to your Apple TV.
The PLEX client rely on a server side (read Mac) component that act as a transcoding helper for all video served to the iOS devices. In some ways it's doing a similar job as the iPhone and iPad AirVideo server side application does… but in a much fancier Apple TV friendly way.
Here's a short video of PLEX client for Apple TV in action:
More Info: Installation Instructions