The untethered jailbreak is a backup plan to the userland method that is intended to be released soon by Comex. From the tweets:
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)
ipt4g 4.2.1 untethered JB w/4.2b3 kernel: http://is.gd/inE4e … Wastes no exploits, but upcoming @comex JB maybe better
The method utilizes iOS 4.2 Beta 3 blobs that have been saved by Cydia on iPhone, iPad and the iPod Touch. We aren’t aware when this is to be released but you can anticipate it to be big. In the meantime you can have a look at the video below to see this in action [RP].