Unlock iPhone 4.0 Beta with ultrasn0w on iPhone 3G
After Redsn0w 0.9.5, iPhone 3G can now be unlocked with Ultrasn0w using Fuzzyband, the baseband downgrader. Unlock iPhone 4.0 Beta on iPhone 3G (only this tested so far) by downgrading baseband to 04.26.08 for ultrasn0w to unlock.
Taimur of RedmondPie has posted a complete tutorial on how to downgrade baseband to 04.26.08 with Fuzzyband and unlock iPhone 4.0 on iPhone 3G.
For complete instructions, head over to his blog on how to unlock iPhone OS 4.0 on iPhone 3G.