This tells you how jailbreak, install nito, and XBMC
http://www.pinglio.com/2011/05/how-to-jailbreak-apple-tv-2-with-seas0npass/
Go here for more detailed jailbreaking instructions and the PC app:
http://support.firecore.com/entries/387605
Be sure to read all the print between screen shots while jailbreaking. I missed an important note (not having the Apple TV plugged in to power when jailbreaking) that made me have to do it again.
After jailbreaking, I would recommend installing nito, and use that to install XBMC rather than installing XBMC directly. Nito is a nice utility to have on your Apple TV
-BrewCoug