The Chronic Dev Team has unleashed the the untethered greenpois0n jailbreak for iOS 4.2.1, which means you don't have to re-root your phone after every reboot.
This time 'round, the program will let you perform an untethered jailbreak -- you can finally reboot the device without having to re-jailbreak. Be warned though -- this build was rushed out and we haven't tested any devices on our end. At the time of this writing, the software is only available for Mac. Oh, and unlockers, don't update to 4.2.1 if you plan on swapping sim cards in the future. Hit the source link to get started.
As always, back up your phone beforehand, and don't proceed unless you really know what you're doing.