Today Ultrasn0w got updated to 1.2.1 o fix issues on iPhone 4/ 3GS running iOS 4.3.1. The issue was that people facing signal break downs and Ultrasn0w was broken on iOS 4.3.1. So today MuscleNerd updated ultrasnow to work on iOS 4.3.1 without using Ultrasn0w Fixer.
Below are the steps mentioned on how to install Ultrasn0w 1.2.1 and fix your issues. Remember this is not a new Unlock for iPhone 4. This is only for old basebands which are 1.59.00 and 6.15.00 and below.
Steps to Follow:
If you have updated to iOS 4.3.1 and got it Jailbreaked, so open Cydia and go to changes. There you will see an Update for Ultrasn0w.
Once done Reboot your Device and you will be done.