Ultrasn0w has been updated to version 1.2.3 with support for preserved baseband running iOS 4.3.3. In other words, ultrasn0w 1.2.3 does not unlock new baseband, but it does work to keep your unlock if you have been tugging along iPhone 4 with 1.59.00 or iPhone 3G & 3GS with 4.26.08, 5.11.07, 5.12.01, 5.13.04, and 6.15.00.

If this applies to you, you’ll want to use PwnageTool 4.3.3 (Mac) or Sn0wbreeze (Windows) to jailbreak while preserving your old baseband and updating to iOS 4.3.3.

Install Ultrasn0w Unlock for 4.3.3
After you are jailbroken, ultrasn0w is installed through Cydia. You can generally do this just by searching for “ultrasn0w” but if that fails:

Add http://repo666.ultrasn0w.com as a source repository to Cydia
Search for “Ultrasn0w 1.2.3″ and install
Reboot your iPhone
Assuming you have supported baseband and everything is done correctly, the iPhone will boot unlocked and ready for another SIM card. Remember that many unofficial carriers do not support 3G networks so you should disable that in network settings if things aren’t working as expected.