The iPhone Dev Team has released the next version of their famous unlock tool, that is Ultrasn0w 1.2. You could use download and install Ultrasn0w 1.2 from cydia in order to unlock your device on iOS 4.2.1 firmware. You would first have to jailbreak iOS 4.2.1 while updating to baseband 06.15 using PwnageTool or Redsn0w so as to obtain Cydia and then install Ultrasn0w 1.2.

Ultrasn0w 1.2 Unlock

But if you upgrade to 06.15 baseband, then you would have to face these :

  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).

Ultrasn0w 1.2 will be able to unlock any baseband versions including iOS 4.2.1 baseband.

Stay tuned to Tips Needed for more jailbreak and unlock news and tips!