You can already stream movies directly from your computer to your apple tv with airplay. They just have to be in iTunes (no avi or mkv) =/
But with StreamToMe you don't need to use iTunes and it doesn't matter what video format you have, just pickup the iPhone/iPad and push the videos to your AppleTV.
So if you've got tons of divx movies that you wanted to watch on your TV, you can do that now via Airplay+StreamToMe.