These guys already answered your initial question.
But I have two Apple TV's: one jailbroken and one not.
The Jailbroken one is cool. I only installed XBMC on it to access files on the network and it works fine. But I am not particularly a fan of XBMC.
The one that isn't jailbroken, I downloaded Air Video and just use that to stream files from my network and that works fine most of the time. You have to have an iOS device and purchase the App (which is 1 or 2 dollars, I think) and it's still rather buggy. I only have problems every once in a while, though.
I recommend the latter, if all you want to do is stream files from your hard drives. But if you want any of the other benefits of jailbreaking.. then obviously that is the route to take.