The actual program that UltraSn0w injects into the baseband to accomplish the unlock is the exact same one the Dev Team developed for YellowSn0w. What changed is that the Dev Team found a new “hole” to inject the unlocking payload. 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 PawnageTool soon.
Please bear in mind, this is no baseband downgrade needed. You can just upgrade to 3.0 tomorrow and wait for the Dev Team to push its unlocking tools and UltraSn0w on Friday. Note that this unlock is only valid for the iPhone 3G. According to the Dev Team, this hack may be applicable to the new iPhone 3G S if it can be jailbroken, which should happen sometimes soon.