iPhone Dev-Team has released a special version of PwnageTool dubbed as PwnageTool 4.1.3 Unlock Edition. You can use PwnageTool 4.1.3 to upgrade your iPhone 3GS/3G baseband to 06.15.00. Note that this PwnageTool release is only for those iPhone 3G/3GS users who are stuck on 05.14 or 05.15 baseband and want to unlock their device with Ultrasn0w.
For those who don't know, Ultrasn0w v1.2 can not unlock iPhone 3G/3GS 05.14.xx or 05.15.xx basebands. So inorder to unlock your device, you should first update your baseband to 06.15.00 by using PwnageTool 4.1.3 and then you can use Ultrasn0w to unlock 06.15.00 baseband. If you are Windows users, you need to wait for updated version of Redsn0w tool which will help you to update your iPhone baseband to 06.15.00.
Download PwnageTool 4.1.3 - Unlock Edition
Update1: Guide to Update iPhone 3GS/3G Baseband 05.14 or 05.15 to 06.15.00 on Mac
Update2: Instead of using PwnageTool 4.1.3, now you can use Redsn0w 0.9.6b5 to update iPhone 3G/3GS baseband from 05.14/05.15 to 06.15.00. It don't require you to download iPad iOS 3.2.2 IPSW plus its available for both Windows and Mac OS X users.