Hi,
I was just discussing this with a chap at work today.
Looking on the apple tv webpage at apple.com, it says it'll stream anything in your itunes library (video or music) from your mac.
This means as long as you can play it in itunes, (h.264, m4v, mov etc,) It'll play on apple tv (as long as the resolution is ok). Well... I don't *have* an apple tv, so I can't say for sure, but that's how it sounds on the website.
here's what I do at the moment with my iphone...it'd be the same deal to stream to the apple tv.
1) Download a divx/mkv etc file, from a totally legitimate source, breaking no laws.
2) use visualhub (google it) to convert whatever file it is to itunes (profiles are setup for iphone/ipodtouch/apple tv etc.)
3) once converted, add to your library, change the settings to your desires (is it a tv show, is it a movie etc. what season is it etc)
4) Enjoy!
Someone with an apple tv, feel free to chime in on if this is correct? I believe it is, and am actually thinking of buying an apple tv next month!