It's haxored. http://code.google.com/p/atvusb-creator/ unlocks the rigorous protections (user: frontrow password: frontrow) and enables SSH. It also installs Boxee.

With SSH it's fairly straightforward to enable USB mass storage, and I currently have an external drive hooked up to the USB port. Making it run Linux is probably easiest by taking it apart and installing off your computer onto the hard drive.

Using Boxee on my AppleTV is like living in the future. Seamlessly plays my torrented files and streams hulu (with netflix Coming Real Soon).

It's fantastic.