The shiny new Boxee Beta is out for every platform except Apple TV, the platform we first used to cut out a cable bill. Some intrepid hackers have, however, created an unofficial installation method for that shiny, slim box.
Following the guide isn't a beginner's task—you'll need SFTP access to your Apple TV, an SSH-capable terminal to work through, and be comfortable enough with either of them to navigate to your /Users/frontrow directory to do the installation work. That said, the process itself is just a step-by-step process for any Apple TV that was already opened up by a previous Boxee installation, and it doesn't wipe out any media that you've got stored on your box at the moment.
Hit the link for the full walkthrough of a Boxee Beta installation on Apple TV. Need motivation? Check out our screenshot tour of the Beta to see what you could be streaming and exploring from your couch.
[FAQ] Boxee Beta on ATV - A Step by Step Guide! [Boxee Forum via Apple TV Hacks]