iPhone Dev-Team have just pushed UltraSn0w version 1.2.1 out to the public! As we mentioned earlier this update does not add support for any new baseband versions it was updated simply so users on the iOS 4.3.1 firmware can use UltraSn0w to maintain their current unlock.
The official word from the Dev-Team below:
We’ve pushed out the 4.3.1 compatibility fix for ultrasn0w in Cydia — it’s now at version 1.2.1. If you’re not already at 4.3.1 and you need the unlock, please be sure you understand how to get to 4.3.1 using a custom IPSW that doesn’t update your baseband. There are lots of guides for this (like at iClarified.com).
This isn’t a new unlock! It’s to allow those who are already unlocked 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.
For those of you who know the drill you can find UltraSn0w 1.2.1 via Cydia!