We had talked about iOS 5 and what should we expect from it, so that you get a rough idea of what Jailbreaking can offer. There was a lot of speculation regarding GreenPois0n bringing the Untethered Jailbreak for iOS 4.2.1 , but actually the RedSn0w team got it about a month before them!!
The Chronic Dev Team who has worked hard on the development of GreenPoison RC5, has finally completed their promise of creating a tool for UnTethered Jailbreaking iOS 4.2.1
You can also note that the latest version from GreenPois0n does not require users to have iOS 4.2b3 SHSH blobs to jailbreak the device. It works well without it!!
We do plan to post the Step-By-Step Jailbreaking guide using GreenPois0n, but it may take some time. Till then you can get your hands dirty trying to Jailbreak iOS 4.2.1 by Downloading GreenPois0n. Its nice that GreenPoison is offering the new Software for all the major Operating Systems namely Mac OS, Windows and Linux.
Note: Windows and Linux users are facing some problems during Untethered Jailbreaking of their iOS 4.2.1 iDevice. (It does not seem to work in most cases)