Seas0nPass has just been released for jailbroken AppleTV users. Up until now, there hasn't been easy to use jailbreak tools for the AppleTV. Hopefully Seas0nPass can change all that.
Seas0nPass offers some of the same basic functionality utilities like PwnageTool already offers iPhone, iPod Touch, and iPad users. It will currently jailbreak the most current version of AppleTV iOS (4.2.1). Seas0nPass for AppleTV also offers an easier tethered boot option as well as an ability to create and restore from custom firmwares (IPSW) files. This will be nice for users on newer firmwares that may not offer an untethered option yet.
Currently Seas0nPass is only available for Mac users running Mac OS 10.6 or later but a Windows version should be coming soon.