iPhone 3G and iPhone 3GS users has been lucky to have carrier unlock by upgrading their baseband to iPad 06.15.00 baseband using Redsn0w tool. However one prominent issue of using iPad 06.15.00 baseband , killing your iPhone GPS.
Number of users has reported that after flashing iPad baseband they no longer get the GPS signal but unfortunately it has been impossible to fix this issue. But not ay more today finally iPhone Dev Team has released updated version of Redsn0w 0.9.14b1 with support for baseband downgrade for iPhone 3G/3Gs having iPad baseband.
Another issue of using iPad baseband is you can no longer use stock firmware but with Redsn0w 0.9.14b1 you can now downgrade specifically from 06.15 to 05.13.04 and have best of bothultrasn0w compatibility and a normal iPhone baseband with full GPS and the ability to use stock IPSWs again.
You can also update to iPad baseband to for the sake of downgrading to 05.13.04 unlockable baseband is also possible for iPhone 3G owners but for iPhone 3GS it depends on date of manufacturing of their iPhone 3GS.
The one manufactured after 34 week of 2011 are not compatible with iPad baseband. so flashing to ipad baseband will brick your iPhone 3GS making it impossible to downgrade or upgrade.
Download Redsn0w 0.9.14b1 with Baseband Downgrade Support
Download Redsn0w 0.9.14b1 for OS X.
Download Redsn0w 0.9.14b1 for Windows.