If your iPhone 3G or 3GS is depend on the ultrasn0w unlock, then you should not update your iPhone directly to a newer version of iPhone firmware through iTunes as the baseband will also be updated. Since the ultrasn0w unlock only works on the iPhone baseband 04.26.08 that is on firmware 3.0/3.0.1, you should avoid it to be updated when you’re restoring to iPhone 3.1/3.1.2 software update as its baseband 05.11.07 currently could not be unlocked through the software unlock solution.
To avoid the baseband being updated, you can update your iPhone with 3.1/3.1.2 custom firmware through iTunes. If you’re on Mac, download pwnagetool 3.1.4 released by iPhone dev team to create the custom firmware for your iPhone 3G or 3GS. As I’m on Windows Vista, I could not show you the steps, but you should be able to follow winandmac.com’s guide that shows how you can use pwnagetool to create custom firmware to jailbreak iPhone 3GS (and others) firmware 3.1.
P/S: You can create 3.1.2 custom firmware through the guide, just use iPhone firmware 3.1.2 instead of 3.1. And make sure you’re using the correct firmwares for your iPhone.
