It was initially expected that the untethered iOS 4.2.1 jailbreak might be released somewhere around the Christmas Eve. But if this latest tweet by Comex is anything to go by, we may not see an untethered jailbreak for iOS 4.2.1 by this Christmas.

I think I’m going to miss my Christmas deadline…

It is to be noted however that the iPhone Dev-Team already has a backup plan ready, but that will require iOS 4.2b3 SHSH blobs for untethered jailbreak. Comex is said to have a more efficient userland-based untethered jailbreak method for iOS 4.2.1. This untethered solution will almost certainly be applicable to all current iOS 4.2.1 devices, including the new A4-based ones which includes iPhone 4, iPad, Apple TV 2G and iPod touch 4G.
More info as it comes, stay tuned !