Greenpois0n RC5 has just been released and it jailbreaks all iOS devices on firmware 4.2.1 and of course its an untethered jailbreak. Shout out to the Chronic Dev Team for all their hard work for actually doing this work on their own without the help of the iPhone Dev Team.At the moment GreenPois0n does not hactivate your idevice, meaning you have to have your iphone past the connect to itunes screen before you can jailbreak.
Fully detailed jailbreak tutorials are coming soon here on theiphonespot. Unfortunately the first release is for Mac OSX only. You can download the updated Greenpois0n RC5 from http://greenpois0n.com/.