Seas0nPass has been updated to untether iOS 4.2.1 JailBreak on Apple TV. As for now , this version only has support for Mac but it will be available for Windows soon. It uses the same exploit by Chronic Dev Team for the untether which is also present in Greenpois0n and Pwnage tool (Dev Team). However , the JailBreak doesn’t provide much use for the Apple TV except the root access but it has been assumed that Apple may allow Apps and Games to run on it. As soon as the feature hits the Apple TV , people will be able to run the cracked versions too.
If you have already JailBroken the device with Greenpois0n RC 6.1 , you dn’t need to re-Jailbreak to untether it.
The updated version and guide for the user are present on : Seas0nPass.com