The iPhone Dev-Team released RedSn0w 0.9.14b2 with improved algorithm for baseband downgrade from the iPad’s 06.15 baseband to 05.13 on the iPhone 3GS and iPhone 3G. The 06.15 baseband was used by many users to unlock their device. Now it is possible to to get back true GPS and an ultrasn0w unlock.
You can download RedSn0w 0.9.14b2 .
Here is additional info from DevTeam: