How to Install Ultrasn0w Fixer on iOS 4.3.1
STEP 1. Launch Cydia on your iPhone
STEP 2. Now simply tap on Manage tab, then tap on Sources.
STEP 3. Now you have to tap on Edit button, the tap on Add button.
STEP 4. Now simply type http://djayb6.com/ as Cydia/APT URL and the tap on Add Source.
STEP 5. Now Cydia will download and update its repository list.
STEP 6. Now tap on Search tab, then type Ultrasn0w Fixer in the search field.
STEP 7. To install Ultrasn0w Fixer on your iPhone, simply tap on Install button.
That’s it. In this way you can unlock iPhone 3GS on iOS 4.3.1. You should note that you have to jailbreak your iPhone 3GS on iOS 4.3.1 using PwnageTool bundles to use Ultrasn0w Fixer for 4.3.1. You can follow the steps posted here to jailbreak iPhone 3GS on iOS 4.3.1 using PwnageTool bundles. [Via]