greenpois0n

Greenpois0n untethered jailbreak for iOS 4.2.1 released
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.

Some provisos, though: the jailbreak is only available for Mac users right now, and the greenpois0n website seems to be down, so you can't actually download the jailbreak. Also, according to the excited Engadget commenters, it sounds like the installation of the jailbreak is a little bit finicky.

As always, back up your phone beforehand, and don't proceed unless you really know what you're doing.