Few days ago, Steve developer of pushfix app, was released a unofficial Blacksn0w RC2 which was patched version of Geohot’s unlock tool for iPhone 3.1.2.
You can unlock iOS 4.0 on iPhone 3GS with Blacksn0w RC 2 Final . Blacksn0w RC 2 works only iPhone 3GS with iOS 4.0 and baseband 05.11.07 hence you must first jailbreak iOS 4.0 using pwnagetool bundles released by msft.guy and then unlock with Blacksn0w RC.
How to Unlock iOS 4 on iPhone 3GS with Blacksn0w RC2 Final
1. First you need to Jailbreak iOS 4.0 on iPhone 3GS using pwnagetool bundles.
2. When you have jailbreaked your iPhone 3GS, use blacksn0w RC2 to unlock iOS 4.0.
3. Start Cydia and Open Manage tabs
4. Now select sources and click on edit and then add, You will be prompted to enter url source, now type “http://cydia.pushfix.info” and click on “Add Source”.
5. Once you have added source wait for Cydia to update packages
6. Now search for Blacksn0w and from the list click on “Blacksn0w RC2 Final” and click on Install.
7. Restart your iPhone and enjoy unlocked iOS 4.0, it may take about 80 seconds to boot & cell connection establishment on iPhone 3GS running iOS 4.0 unlocked with blacksn0w RC2 Final.