iPhone Dev team has released an updated version of ultrasn0w to unlock iPhone 4. With ultrasn0w, you can free your iPhone to use with any carrier. The latest version of ultrasn0w supports carrier unlock for iPhone 4 with baseband 01.59.00, as well as, 3G and 3GS riding on baseband 04.26.08, 05.11.07, 05.12.01 and 05.13.04.

Note for T-mobile USA user: Please ensure you disable 3G service before installing ultrasn0w.

To Unlock iPhone 4, just follow these procedures:

Step 1: Upgrade your iTunes to iTunes 9.2 version or latest and backup your iPhone data by synchronizing it with iTunes 9.2.

Step 2: Use PwnageTool or Redsn0w or JailbreakMe to jailbreak your iPhone 4.

Step 3: Launch Cydia on iPhone and add “http://repo666.ultrasn0w.com” as the new repository URL. You can simply tap “Manage” -> “Sources” -> “Edit” -> “Add” to add the repository URL. If your not clear about this, checkout my post here on how to add sources to Cydia.

Step 4: Then tap “Search” and search for “ultrasn0w” application
Step 5: Install “ultrasn0w”

Step 6: Once the installation completes, reboot your iPhone. Your iPhone is unlocked and you are free to choose other carriers.