The Chronic Dev Team has released the greenpois0n new version RC6 which will allow you to performs untethered jailbreak of iOS 4.2.1 and allows you to install nitoTV on your Apple TV. This jailbreak is applicable to any Apple device running iOS 4.2.1 like iPad, iPhone or iPod Touch.
This new release includes a loader that will load nitoTV to your Apple TV. There are other fixes over RC5, in RC6 release the emulation problems have been fixed
Greenpois0n RC6 is now available for both Windows and Mac on their website. If the site may is not loading, you can always search for alternative mirrors in Google.
Note: Some of the jailbreakers are reporting to have encountered some issues on WiFi, booting etc with RC6.