iPhone Dev Team member, MuscleNerd has confirmed Ultrasn0w 1.2.1 unlock for iOS 4.3.1. The unlock 4.3.1 is compatible with iPhone 4 and iPhone 3GS. Remember the Ultrsn0w 1.2.1 DO NOT unlock higher basebands whose unlock is not possible right now. But it will continue to unlock the old ones.
You can say that its just a new version of Ultrasn0w for people who have upgraded to iOS 4.3.1 and also jailbroken it using PwnageTool 4.3 or Sn0wbreeze 2.5. These tools can upgrade you to iOS 4.3.1 firmware while preserving your baseband.
If you are still confused whether its compatible with your baseband or not, it can ONLY be compatible with basebands which includes 01.59.00 / 04.26.08 / 05.11.07 / 05.12.01 / 05.13.04 / 06.15.00.
Previously we have updated you about Ultrasn0w fixer for iOS 4.3.1. But few of the users are still complaining minor issues they face after using it. Ultrasn0w 1.2.1 for 4.3.1 unlock will resolve all the issues.
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