These are instructions on how to downgrade your baseband from 2.30 to 2.28 for iPhone 3Gs with the 5.8 bootloader. This will allow you to unlock your iPhone 3G using YellowSn0w. An similar exploit is still be worked on for those with a higher bootloader version...
IMPORTANT: Make sure your iPhone uses the 5.8 bootloader before continuing. You can find instructions on how to check your bootloader version here: Windows, Mac. Ensure you are on the 2.30 baseband by following this tutorial. You will also need MobileTerminal installed to completed this tutorial.
Launch Cydia by pressings its icon on your SpringBoard.
Press to select the Sections tab at the bottom of the screen.
Press to select iClarifed from the list of Sections. If you don't see iClarified you will need to add the iClarified source using these instructions.
Press to select DownBB from the list of Packages.
Press the Install button at the top right of the screen.
Press the Confirm button to begin installation.
Once installation has completed successfully press the large Return to Cydia button.
Press the Home button to return to your SpringBoard then press to launch Terminal
Enter su into the terminal window then enter alpine when prompted for a password.
Enter DownBB into the terminal window to launch the baseband downgrade script.
You will be asked to confirm you would like to continue. Please make sure you are on the 2.30 baseband and have the 5.8 bootloader before entering yes into the window.
DownBB will now execute the script which downgrades your baseband.
Once the script has completed it will automatically reboot your iPhone.
Please note you will now be on the 2.28 baseband.
You may now unlock your iPhone using the iPhone 3G unlock tutorial found here!
Many thanks to EvilPenguin|, pH, and DHowett for their work on this script! Thanks to Geo-hot for discovering the exploit!