As expected the Dev Team released tonight an updated version of their UltraSn0w unlock for iPhone 4.3.1. As we noted before, this version of UltraSn0w will not unlock any new baseband. UltraSn0w 1.2.1 will unlock the iPhone 3GS or iPhone 4 on baseband 01.59.00, 04.26.08, 05.11.07, 05.12.01, 05.13.04, and 06.15.00.

If you are on any of these basebands, you can update to iOS 4.3.1 while preserving your baseband using PwnageTool. If your baseband is different from the one listed above, I’m afraid there is no unlock for you at this time…

From the Dev Team blog:

This isn’t a new unlock!  It’s to allow those who are already using ultrasn0w to use FW 4.3.1. It also fixes the signal bar issue for those who aren’t using the unlock but retain an older baseband intentionally.

AFTER INSTALLING ULTRASN0W 1.2.1, PLEASE REBOOT YOUR iPHONE using the normal “slide to power off” swipe.  T-Mobile users in the USA also should disable 3G mode in Settings->General->Network.

You can install UltraSn0w from Cydia by adding this repo: http://repo666.ultrasn0w.com. If you’re not sure how to to that, you can always follow our iPhone 4.3.1 unlock tutorial.