The iPhone 2G and 3G running OS 3.0 can now be jailbreaked thanks to Redsn0w. Please note that the iPhone 3GS cannot be jailbroken just yet.
'redsn0w is an easy to use, multi-platform, multi-device jailbreaking and unlocking (iPhone 2G only) tool for the iPhone 2G (original iPhone), the iPhone 3G (but not the 3GS) and also the iPod touch (first and second generation). Currently it is available for Windows and Mac OS X (there are some issues using redsn0w with OS X PPC, please use an Intel Mac until we have this problem resolved).
Please note that there is also now a way to unlock your iPhone 3G by using Ultrasn0w. Details found here.
Redsn0w is available to download as follows...
Instructions can be found here.