Redsn0w 0.9.6rc9, PwnageTool 4.3 and Sn0wbreeze 2.5 were released yesterday to jailbreak iOS 4.3.1 untethered. But unfortunately ultrasn0w is broken. So you should follow this guide to unlock your iPhone 4 and iPhone 3GS.
Step 1: Open Cydia on your iPhone 4 or 3GS
Step 2: Go to Manage->Sources
Step 3: Now tap on “Edit”, then tap on “Add”
Enter the following repo
Now tap on “Add Source”
Step 4: Tap on "Done".
Step 5: Tap on "Pushfix.info" and install
Step 6: After the installation Go to Pushfix.info again. Now scroll down and look for “Ultrasn0w fixer for 4.3.1” Tap on it. Install the package. It will ask you to reboot your device.
Now your iPhone 4 and iPhone 3GS on iOS 4.3.1 is unlocked on older basebands. Remember,
Note: It is available for iPhone 4 and iPhone 3GS only.
if you are facing a problem on iPhone 4 and iPhone 3GS after jailbreak iOS 4.3.1, the problem was No service after ultrasn0w 1.2 baseband 06.15.00. Now you can fix this problem by Clicking Here