The Iphone dev team has come up with instructions on jailbreaking the Iphone 3.0 . This uses redsn0w â€” the multi-platform, multi-device jailbreaking and unlocking tool. Redsn0w only supports the iPhone 2G (original iPhone), iPhone 3G and the iPod touch (first and second generation) for the time being. The latest iPhone 3G S is not yet supported for jailbreaking or unlocking so don’t try to use it on your brand new phone! Jailbreaking is supported across all devices running OS 3.0, except for the 3G S, and unlocking is only available for the iPhone 2G. 3G owners need to wait for the upcoming ultrasn0w to be released in order to unlock their OS 3.0 devices.
Just head over to their Blogpost on it. This does not apply on IPhone 3Gs.