iPhone Dev Team member, MuscleNerd has confirmed the arrival of Ultrasn0w 1.2.1 unlock for iOS 4.3.1. The unlock 4.3.1 will be compatible with iPhone 4 and iPhone 3GS. Remember the Ultrsn0w 1.2.1 WILL NOT unlock higher basebands whose unlock is not possible right now. But it will continue to unlock the old ones. Ultrasn0w 1.2.1 will be released tonight.
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 will 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.



This is what MuscleNerd has updated on his Twiiter account:
4.3.1-compat ultrasn0w update for *current unlockers* is out to external testers. Barring any hiccups, release is tonight
We will update you as soon as they release Ultrasn0w 1.2.1 unlock 4.3.1.