Fuzzyband, the application that will downgrade your baseband with a tap of a button, was updated and now it supports iPhone OS 4.0beta1. Theoretically you can downgrade your baseband, and use ultrasn0w to unlock your iPhone 3G running OS 4.0beta1. Why theoretically? Because it will work only for 05.08 bootloader users – early iPhones , which are pretty rare this days…
You can find Fuzzyband for free in Cydia via BigBoss repo…