Finally, Greenpois0n RC5 is out for iOS 4.2.1. This jailbreak is an untethered jailbreak i.e. user can finally reboot their devices without having to re-jailbreak.Previously, whenever user reboot their devices, it goes into DFU mode and it is required to again jailbreak it, to bring it back to working condition.
Those people who are using ultrasnow for network unlock must stay away from this, because to jailbreak iOS4.2.1, it needs a fresh installation of iOS 4.2.1 from iTunes, that results into updating users device baseband and it will end with locked iPhone 4.
Chronic dev team has released the version of Greenpois0n for Mac, windows version is expected very soon, the mac version can be downloaded from the official site.
Download GreenPois0n RC5 for Mac OS X from here