After releasing Redsn0w and PwnageTool 4.3 for untethered iOS 4.3.1 jailbreak, now iPhone Dev Team pushed Ultrasn0w 1.2.1 unlock for iOS 4.3.1 that includes fix for signal bar issues.

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.

Check also:
How to jailbreak iOS 4.3.1 Untethered with Sn0wBreeze 2.5.1, Redsn0w 0.9.6 RC12 Untethered iOS 4.3.1 Jailbreak, How to Use PwnageTool 4.3 for Untethered iOS 4.3.1 Jailbreak

Basebands Supported by Ultrasn0w 1.2.1 Unlock

iPhone 3G/3GS

• 04.26.08
• 05.11.07
• 05.12.01
• 05.12.04
• 06.15.00

iPhone 4

• 01.59.00

How to Unlock iOS 4.3.1 with Ultrasn0w 1.2.1

Requirement:
Jailbreak your iPhone 4, 3GS, 3G by preserving the old baseband with PwnageTool (Mac), or Sn0wbreeze (Windows).

Step 1

Start Cydia on your iOS 4.3.1 jailbroken iPhone

Step 2

Tap on Manage tab > Sources

Step 3

Tap Edit > Add and type this URL http://repo666.ultrasn0w.com to add the repository and tap on Add Source

Step 4

Cydia will start auto update process, so let it complete

Step 5

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.