The only way to achieve an untethered jailbreak on iOS 4.2.1 as yet is through Redsn0w 0.9.7, which however has limitations as it required you to have SHSH blobs of iOS4.2b3 saved on Cydia. Now, the iPhone Dev-Team has updated Redsn0w and ported back the Monte technique to the iOS 4.1 kernel.
As mentioned above, previously for an untether jailbreak using Monte, you had to have SHSH blobs of iOS4.2b3 saved on Cydia which happens to be developer only, however now after this update, you shall only need SHSH blobs of iOS 4.1.
The iPhone Dev-Team has also revealed that it plans on porting forward the Monte technique to the recently seeded iOS 4.3.
This is what was tweeted using the @redsn0w_testers account:
Ported Monte back to 4.1 kernel (SHSH blobs). Next step: porting it forward to 4.3beta!
According to a previous report, it was said that the iPhone Dev-Team was delaying the release of the Windows version of Redsn0w as it had to fix performance issues, and it looks like the next version with these new capabilities will be released for both platforms, Windows and Mac.
Stay tuned for more information, we shall definitely keep you updated!
Did you know: The Chronic Dev Team is also going to release a new version of GreenPois0n which will be untethered for iOS 4.2.1 upon release and they have also recently demoed it on video.