The key member of iPhone Dev team, MuscleNerd has confirmed that they have fixed the Ultrasn0w issues with iOS 4.3.1 and an updated version of Ultrasn0w unlock will be released soon to fix signal bar issues on iOS 4.3.1 compatible iPhones running on the old baseband.
MuscleNerd: The 4.3.1 issues (signal bars, etc) are fixed http://is.gd/OYktXf but need a few days to test all devices + FW (again: not a new unlock!)
The official ultrasn0w 1.3 / 1.2.1 repo http://repo666.ultrasn0w.com will cover iPhone 4 baseband 01.59.00, iPhone 3GS basebands 04.26.08, 05.11.07, 05.12.01, 05.13.04 including06.15.00. Baseband 05.14.02 and 05.15.04 will be still locked and you need to update your iPhone 3GS to baseband 06.15.00. However, you will lose your GPS (known issue) if you install the iPad (6.15.00) baseband. Currently, there is no unlocking for iPhone 4 baseband 2.10.01, 3.10.01, 4.10.01.
MuscleNerd: It’s probably confusing, so worth repeating: this isn’t a new unlock. It’s a compatibility fix for current unlockers only
MuscleNerd: (and for those retaining older baseband for resale value even though they don’t use the unlock themselves)
Ultrasn0w 1.3 will support iPhone 4 baseband 01.59.00, iPhone 3GS basebands 04.26.08, 05.11.07, 05.12.01, 05.13.04, 06.15.00. You should note that this updated version of Ultrasn0w will not be a new unlock for iPhones with newer basebands.