Unlike its predecessor YellowSn0w,UltraSn0w will unlock your iPhone 3G no matter what bootloader or baseband you have, as long as you have the new 3.0 firmware installed.
During his demo, MuscleNerd successfully jailbroke his iPhone 3G and proved it by inserting a T-Mobile SIM card.UltraSn0w won’t be available until Friday and like YellowSn0w, it will be downloadable through Cydia. Your iPhone will obviously have to be jailbroken and the Dev Team will release the necessary tools for that (QuickPwn and PwnageTool) soon.
NOTE: UltraSn0w is only valid for the iPhone 3G. According to the Dev Team, this hack may be applicable to the new iPhone 3Gs if it can be jailbroken, which should happen sometimes soon.