The Chronic Dev-Team has just released the latest version of Greenpois0n, which features an untethered jailbreak for iOS 4.2.1 without the need for SHSH blobs. The exploit they are using has already been patched in iOS 4.3.
Currently it is Mac only and is a beta release. A Windows version will be released "soon." You can download the jailbreak from www.greenpois0n.com
You can find our tutorials on how to jailbreak with Greenpois0n available at above link also.:
Window version also available, servers being hammered at moment so you might have to wait awhile.
Update its the RC5 release u will need.