The iPhone Dev team today officially released ultrasn0w which allows you to SIM-unlock any iPhone3G (not 3GS) running iPhone OS 3.0. iClarified has a nice tutorial on the installation which we’ve shamelessly pasted below.
These are instructions on how to unlock the iPhone 3G for use with any GSM cellphone carrier using UltraSn0w.
Before you can follow these instructions you must have a jailbroken iPhone and you must be on the 04.26.08 baseband(modem firmware). This means that you must be running the 3.0 firmware and have used PwnageTool or RedSn0w to jailbreak.
To find your firmware and modem firmware(baseband) versions you can follow this tutorial. If you are not on baseband version 04.26.08 then you need to follow one of these tutorials before unlocking: Mac, Windows
If you are on T-Mobile remember to turn off 3G before starting…
For your convenience this unlock tutorial is available in video format also. Click the Watch It button to view the tutorial!
Now press the Home button then power off and power on the iPhone. You do this by holding down the power button for 3 seconds then moving the power slider that appears to the right. Press the power button again to turn on the phone.
***THANKS: A big thank you to all the members of the iPhone Dev-Team for all their hard work! It is much appreciated by all.