Just a moment ago, Chronic Dev team kick out the RC6 version of GreenPois0n to untethered Jailbreak the Apple TV on iOS 4.2.1. RC6 got some fixes so its important to update it, pod2g tweets, you can apply RC6 over RC5 without restoring.

greenpois0n RC6

Download GreenPois0n RC6 for Windows

Download GreenPois0n RC6 for Mac