How to Downgrade Your iPhone 3G Baseband Using Fuzzyband
These are instructions on how to downgrade your baseband to 2.28 for iPhone 3Gs with the 5.8 bootloader using Fuzzyband. This will allow you to unlock your iPhone 3G using YellowSn0w.
Fuzzyband is the preferred application for downgrading your baseband since it does version checking, downgrades all versions of baseband from 2.30.03 to 4.26.08 (3.0 beta5), and it reports the current bootloader version on screen.