The shiny new Boxee Beta is out for every platform except Apple TV. 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.