Few days back, the GreenPois0n RC5 beta versions for untethered jailbreak of iOS 4.2.1 was released by the Chronic Dev Team. Now they have come up with the GreenPoison RC6, for both Windows and Mac OS X, which could be used to jailbreak Apple TV 2G on iOS 4.2.1. GreenPoison RC6 supports untethered jailbreak on the Apple TV 2G.
You could download GreenPois0n RC6 for Windows and Mac OS X from the below links :
Download GreenPois0n RC6 for Windows from here
Download GreenPois0n RC6 for Mac OS X from here