iPhone Dev team has released an updated version of Ultrasn0w – v1.2.1 to fix the iOS 4.3.1 compatibility issues and to also the signal bar issue reported by users who had jailbroken their iPhone using PwnageTool, Redsn0w or Sn0wbreeze for iOS 4.3.1.
Ultrasn0w 1.2.1 for iOS 4.3.1 currently supports iPhone 4 baseband 01.59.00 and iPhone 3GS basebands 04.26.08, 05.11.07, 05.12.01, 05.13.04 and 06.15.00.