Those who have jailbroken iPhone 4.2 (iPhone jb guide here) or iPad 4.2 (iPad jb guide here) know by now that it's a tethered jailbreak. Meaning, you must plug your device in every time you reboot it for any reason.
The good news is that the Dev-Team has finally managed to run an untethered jailbreak on iOS 4.2.1 for those with their 4.2b3 hashes saved. Here's the official word from the Dev-Team's MuscleNerd via Twitter:
Hopefully we will see more on this soon. We will keep you posted on anything else that comes from the Dev-Team.