If you have your iPhone 3G/3GS flashed to iPad baseband 06.15.00 to retain the ultrasn0w unlock, and now somehow managed to make your phone unbootable here is the solution.

While trying to resurrect an Apple iPhone 3G stuck in a reboot loop at boot logo, I found that the only solution is to go for an factory unlock service and restore to the original iPhone iOS firmware with current modem version. Apple needs the correct baseband/modem version to enable the legal factory unlock via iTunes, and if you have the iPad baseband 06.15.00 it wont work. Here is a tutorial on how to downgrade the iPad baseband 06.15.00 to the correct one on Apple iPhone 3G.

Process to downgrade iPad baseband using redsn0w :

1. Launch redsn0w and click Extras button.
2. Click Select IPSW and point the file-browser to Apple stock firmware currently installed on your device. If you don't have the IPSW file download it from - http://www.felixbruns.de/iPod/firmware/
3. After following all onscreen instructions carefully, Click the back button and press Jailbreak button.
4. You will now see the screen as below, make sure you check "Install Cydia" and "Downgrade from iPad baseband" options.

5. Click next and wait patiently for redsn0w to download required files, this takes quite some time so dont cancel this.

6. Let redsn0w complete the process.

7. You should now have sucessfully downgraded your baseband.