According to a tweet from Comex, news is that you may not need to restore or re-jailbreak your device to get the new iOS 4.2.1 untethered jailbreak, . 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.
Is iOS 4.2.1 Untethered Jailbreak Going to be a Cydia Package?
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Although the Sad part is that Comex's jailbreak is delayed and it won't be released by 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!