The latest Ultrasn0w update is compatible for the basebands which are supported by the previous version of application. The iPhone 4 users must be careful and should prevent the baseband while they upgrade to iOS 4.3.1.
Basebands Supported by Ultrasn0w 1.2.1 Unlock
How to Unlock iOS 4.3.1 with Ultrasn0w 1.2.1
Jailbreak your iPhone 4, 3GS, 3G by preserving the old baseband with PwnageTool (Mac), or Sn0wbreeze (Windows).
Start Cydia on your iOS 4.3.1 jailbroken iPhone
Tap on Manage tab > Sources
Tap Edit > Add and type this URL http://repo666.ultrasn0w.com to add the repository and tap on Add Source
Cydia will start auto update process, so let it complete
In Cydia search for Ultrasn0w 1.2.1 and install it, the application will unlock your iPhone which could be used with any carrier
Just reboot your iPhone as its successfully unlocked over iOS 4.3.1.