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.
So, to recap.. this is a new version of ultrasn0w (1.2.1) that is compatible with iOS 4.3.1 that will unlock your iPhone if it was previously unlocked with ultrasn0w.
Full instructions on how to update to the latest version of ultrasn0w, and how to Jailbreak your device for iOS 4.3.1, can be found at this Redmond Pie tutorial.
The update only works with the following basebands 01.59.00 / 04.26.08 / 05.11.07 / 05.12.01 / 05.13.04 / 06.15.00.
You can upgrade your baseband if you are on iPhone 3GS to 06.15.00 if you follow this tutorial.
Happy hacking!