iPhone Dev-Team has finally released Ultrasn0w 1.2.1 which can unlock iPhone 4 and iPhone 3GS on iOS 4.3.1 running old basebands. If for some reason you do not have an active WiFi connection, you can always unlock your iPhone offline by installing Ultrasn0w 1.2.1 DEB file. This is a simple two-step process which you can accomplish either by SSH'ing (Win | Mac) into your iPhone or by using iPhone Explorer (Without SSH).
Steps to Install Ultrasn0w 1.2.1 on Your iPhone 4/3GS Offline:
NOTE: If you have already installed Ultrasn0w 1.2 on your iPhone, first uninstall it by navigating to Cydia->Manage->Packages->Ultrasn0w->Modify and select Remove option.
Step 1: Download Ultrasn0w 1.2.1 DEB file
Download Ultrasn0w 1.2.1 DEB file from Ultrasn0w.com [Direct Link]
Download Ultrasn0w 1.2.1 DEB file from RapidShare
Download Ultrasn0w 1.2.1 DEB file from MegaUpload
Download Ultrasn0w 1.2.1 DEB file from Deposit Files
Download Ultrasn0w 1.2.1 DEB file from Hotfile
Download Ultrasn0w 1.2.1 DEB file from zSHARE
Download Ultrasn0w 1.2.1 DEB file from Uploading
Step 2: Connect your iPhone to your PC or Mac and launch iPhone Explorer. Place the Ultrasn0w DEB file in AutoInstall directory by navigating to the following location:
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Reboot your iPhone normally using SLIDE TO POWER OFF swipe. If you are a T-Mobile USA user, DO NOT forget to disable 3G mode in Settings->General->Network before rebooting your iPhone. After the reboot, you'll find your iPhone fully unlocked on iOS 4.3.1 with all signal-bars issue fixed.