The guy who unlocked iOS 4.0 Beta 4 , PwnageTool bundles for iPhone 3GS on iOS 4 is back again with a new patch, which makes Blacksn0w unlock compatible with iOS 4.0, provided that you have preserved your baseband to 05.11.07 using PwnageTool.
Description of the file blacksn0w4GM.tgz, by msft.guy:
Patched for 3GS GM; manual load; run /bin/blacksnow.sh after booting. Blacksn0w unlock (C) geohot http://blackra1n.com
How to Unlock iPhone 3GS on iOS4 using Blacksn0w
Step 1: First jailbreak your iPhone 3GS on iOS 4 using PwnageTool bundles on the latest GM release. By this, you will be able to preserve your older 05.11.07 baseband.
Step 2: Download Blacksn0w for iOS 4.0 GM .
Step 3: Follow the instructions as seen in the video below (note: video was originally meant for Beta 4, but applies to this GM release too.) to unlock iPhone 3GS, on iOS 4.0.
Blacksn0w ported to 4.0 in action on a 3GS 4.0 beta 4 – Video
Note: We haven’t tested it yet. But, this is working on a 3GS with 05.11.07 baseband. You will need to reapply the patch every time you reboot your phone. Proceed only if you are an advanced user, and know your iPhone inside out. otherwise wait for 2 weeks! We are not responsible if you end up bricking or locking your phone.