iPhone Dev Team has released the redsn0w 0.9.6 b4 / beta 4 for Windows and Mac OS X to jailbreak iOS 4.2.1 on all the devices. This release is to support the ultrasn0w 1.2 unlock  that was released earlier. Redsn0w 0.9.6 b5 upgrades the baseband to 06.15 which is required to unlock iOS 4.2.1. But you have to face the following problems if you do so.

  1. There is no way to come back down from 06.15, and there’s no hiding the baseband version from Apple.  You’ll be voiding your warranty in a very obvious way.
  2. If some future baseband comes out with a critical fix, you won’t be able to update to it if it remains down in the 05.xx sequence (then again, you wouldn’t update to it if you wanted to keep your unlock anyway).
  3. Starting with FW 4.2.1 if you have 06.15 on your iPhone you won’t ever be able to restore to stock firmware (it will fail).  You’ll need to only restore to custom IPSWs (then again, if you’re unlocker you should already be doing that).

Redsn0w 0.9.6 b5

Download Redsn0w 0.9.6 b5 for Windows from here

Download Redsn0w 0.9.6 b5 for Mac OS X from here