The dev team has released their latest carrier unlock, ultrasn0w, It requires a quick using redsn0w and then the installation of ultrasn0w through Cydia. If you’ve already jailbroken your iPhone 3G simply add repo666.ultrasn0w.com to your repositories list and download ultrasn0w.
What does this do? It carrier unlocks the iPhone 3G. Sadly, it doesn’t yet work on the 3G S. It is also safe to update to the latest 3.0 version.
Quickpwn writes:
It’s safe to update to the current iPhone 3.0 baseband – ultrasn0w will work with it, but for future updates such as firmware 3.0.1 (or whichever # the next firmware is) you should hold off on updating and check here first to see if it’s safe to update. Apple may be able to make a baseband update that permanently locks your firmware just as they did with firmware 2.2.1 users.