After a long wait, Finally Greenpois0n is out for iOS 4.2.1, The good thing about this jailbreak is that it is an untethered jailbreak i.e you can finally reboot the device without having to re-jailbreak. Before that whenever you reboot your device your Device goes into DFU mode and you’ve to again jailbreak it to bring it to working state.
Those of you who are relying on ultrasn0w for network unlock must stay away from this because this to jailbreak iOS4.2.1 you need to first do a fresh installation of iOS 4.2.1 from iTunes resulting will update your baseband and you will end up with locked iPhone 4.
Currently chronicdevteam have only released the mac version of Greenpois0n which can be downloaded from here
The iPhone Help
The iPhone Help