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.
Ultrasn0w 0.92 has just been released that fixes the carrier logo on unlocked iPhone 3.1.2 firmware.
Users who jailbroke their iPhone using PwnageTool 3.1.4 or later, must have noticed that the carrier name went missing after jailbreaking. This was a known bug and the Dev-Team has been quick enough to release a fix for that.