You may don't need to restore or re-jailbreak your device to get the new iOS 4.2.1 untethered jailbreak, according to this tweet from Comex. This means that you'll not need to install Redsn0w and use it to get the untethered jailbreak, it should be added a Cydia package on some repository like BigBoss or ModMyi on Cydia.
comex: it's probably going to be a Cydia package. if I can get it to work.However, comex's untethered jailbreak is delayed and won't be released by the Christmas Eve so you have to take in mind that the iPhone Dev Team has it's own backup plan that requires iOS 4.2 beta 3 SHSH blobs to be able to get the untethered jailbreak. More updates as they come ! Stay tuned !