The iPhone Dev-Team released an update to UltraSn0w that makes it compatible with iOS 4.3.1 for basebands previously unlocked. You can use ultrasn0w if you need carrier unlock and you have compatible baseband, preserved with pwnagetool, sn0wbreeze or tinyumbrella.
Ultrasn0w 1.2.1 supports only the following basebands:
- iPhone 4: 01.59.00
- iPhone 3G/3GS: 04.26.08, 05.11.07, 05.12.01, 05.13.04, 06.15.00.
You can install ultrasn0w via Cydia.
Here are DevTeam comments:

![redsn0w-096b5-top redsn0w 096b5 top Step by step Tutorial: How to Jailbreak and Unlock Your iPhone 3GS Using RedSn0w 0.9.6b5 (Windows) [iOS 4.2.1]](http://ultrasn0w.ru/wp-content/uploads/2010/11/redsn0w-096b5-top.png)
![redsn0w-096b5-top redsn0w 096b5 top Step by step Tutorial: How to Jailbreak and Unlock Your iPhone 3G Using RedSn0w 0.9.6b5 (Windows) [iOS 4.2.1]](http://jailbreak-iphone.ru/wp-content/uploads/2010/11/redsn0w-096b5-top.png)
![pwnagetool413-3gs-3 pwnagetool413 3gs 3 300x267 Step by step Tutorial: How to Jailbreak and Unlock Your iPhone 3GS Using PwnageTool 4.1.3 (Mac OS) [iOS 4.1]](http://ultrasn0w.ru/wp-content/uploads/2010/11/pwnagetool413-3gs-3-300x267.png)
![pwnagetool-413-1 pwnagetool 413 1 500x361 Step by step Tutorial: How to Jailbreak and Unlock Your iPhone 3G Using PwnageTool 4.1.3 (Mac OS) [iOS 4.2.1]](http://ultrasn0w.ru/wp-content/uploads/2010/11/pwnagetool-413-1-500x361.png)