The fireCore team has put together a jailbreak application called Seas0nPass. Seas0nPass requires you to connect your Apple TV to your computer temporarily to boot on it. The jailbreak application then lets you load custom IPSW files on to the Apple TV.
fireCore is the same team that created aTV Flash, software that lets you load Last.fm and an HTML5 web browser to Apple TV. aTV is currently in beta and can be pre-ordered for $19.95. aTV requires a second generation jail-broken Apple TV.
Seas0nPass is open source and is free. It is compatible with Mac OS X 10.6. You can download the software and read the instructions on Seas0nPass.com.