iPhone Dev Team has released Ultrasn0w 1.2.1 unlock for iOS 4.3.1. The unlock tool can be used with iPhone 4 and iPhone 3GS. Its not a new unlock, it can only unlock lower basebands whose unlock was already possible but broken on iOS 4.3.1. Ultrsn0w 1.2.1 WILL NOT unlock higher basebands whose unlock is not possible right now.
If you have upgraded to iOS 4.3.1 firmware and jailbroken you device with PwnageTool 4.3 or Sn0wbreeze 2.5 and requires an unlock then you can use Ultrasn0w 1.2.1. Check out the compatible baseband list below.
Compatible basebands with Ultrasn0w 1.2.1:
iPhone 4: 01.59.00
iPhone 3GS: 04.26.08 / 05.11.07 / 05.12.01 / 05.13.04 / 06.15.00
DO NOT USE WITH the following basebands:
iPhone 4: 02.10.04 / 03.10.01 / 04.10.01
iPhone 3GS: 05.14.02 / 05.15.04 / 05.15.04 / 05.16.01 / 05.16.02



Remember you need to have a jailbroken device with PwnageTool 4.3 and Sn0wbreeze 2.5.