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 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.
A big thanks to @sbingner and @ronaldsb for helping with the testing of this update!