The Chronic Dev team of eager iOS hackers at GreenPois0n has announced an untethered jailbreak for the current Apple TV in the form of RC6 of the eponymous utility. This new build features contributions from the NitoTV team to enable the tetherless feature.
Up until RC6, most iOS 4.2 jailbreak tools for Apple TV performed a tethered jailbreak, meaning you would have to connect your device to your Mac or PC on reboot to get it to start up properly. While this wasn't a huge issue with the ATV (you could simply set it not to sleep), it could be a pain in the neck with iPhones and iPod touches if they lost all charge and needed to be restarted. Past revs of GreenPois0n added untethered JBs for the other devices, but this is the first build that will untether the ATV.
I successfully used Seas0nPass to jailbreak my ATV last week, but I may restore it back to normal to give GreenPois0n a try.
Thanks to everyone who sent this in. Clarified post to distinguish between ATV untethered jailbreak & other devices.