The iPhone Dev Team has released PwnageTool 3.0 supporting the recently released 3.0 OS for the iPhone and iPod Touch by Apple in WWDC 2009 event to jailbreak iPhone 3.0. Pwnage Tool 3.0 cannot be used for jailbreaking the iPhone 3GS, and will only work for Original iPhone (1st Generation), Original iPod touch (1st Generation) and the iPhone 3G.
The iPhone Dev Team has advised not to use the Pwnage Tool 3.0 if your are already using yellowsn0w in your 3G iPhone as it may lock the baseband. For that the Dev Team is going to release ultrasn0w shortly. We are keeping close watch on its release and the latest updates are available here. Meantime you can watch the ultrasn0w demo to unlock iPhone 3G running iPhone OS 3.0 firmware.