I used atvusb-creator to install XBMC & Boxee on my AppleTV.
It’s very easy: create a patch stick with a USB flash drive, power down the AppleTV, insert the flash drive and restart the AppleTV. It will boot from the flash drive. You’ll see Linux startup messages and in a few seconds it will be finished. Shut off the AppleTV, remove the flash drive, and when you restart the AppleTV there will be a new menu item for XBMC & Boxee. You’ll probably need to run the update before using it the first time.
XBMC works beautifully. It lets you play additional media formats not normally supported by the AppleTV, including AVI, and adds some nice new features like weather & news.
Unfortunately I had some difficulty with Boxee – it won’t respond to the remote, so there’s no way to even log in.
Update: Restarting seems to have fixed it. I was able to log into Boxee, but now I found a different problem: it truncates file names on a SMB share, so it can’t open any files from my server.