MuscleNerd recently tweeted, Got "backup plan" for untethered 4.2.1 JB working (video coming up). Uses mysterious 4.2b3 hashes Cydia kept for you.
(It's a "backup plan" because comex is working on a more extensible one, but this method is also generalizable)
We'll keep you up to date with more information as it develops.
Update:
MuscleNerd has confirmed that this will work with AppleTV 2G 4.2.1 too.
Update x2:
Comex has said that we'll get an untethered jailbreak by Christmas.
![iPhone Dev-Team Successfully Untethers iOS 4.2.1 Jailbreak [Video] [Update x2] iPhone Dev-Team Successfully Untethers iOS 4.2.1 Jailbreak [Video] [Update x2]](/images/news/13121/45425/45425-500.png)