Procedure to Unlock iPhone 3GS/3G With blacksn0w
* Once you are done with jailbreaking tour iPhone 3GS or iPhone 3G using the Blackra1n RC3, click the blackra1n app and you will see “Sn0w” in the packages list.
* “Sn0w” is nothing but the blacksn0w which will help you to unlock your iPhone. Just click on it.
* Once you have installed blacksn0w it will automatically unlock your iPhone 3G or iPhone 3GS running 3.1.2 firmware on baseband v05.11.07.
Once you are successful in unlocking iPhone 3G or iPhone 3GS you can use any SIM card of your choice.
Apple Section 
Subject: Unlock iPhone 3GS and 3G With 3.1.2 Firmware and 05.11.07 Baseband Using blacksn0w 
23rd January 2010, 3:17 pm