Android port for iPhone 3G is now finally available for download. You can do pretty much everything that you would expect a normal Android phone to do but it is still not recommended for day-to-day regular use. The power management functions of Android for iPhone 3G are yet to be implemented so your phone will only last for an hour or so of Android use before the battery runs out. Yes, it is good only for showing off to your friends that you have Android running on iPhone 3G.
Similar to the Android port for iPhone 2G, you will have both iPhone OS and Android on your device in dual boot setup. Installation instructions are mostly similar to the iPhone 2G one. You will need a jailbroken iPhone to get this to work.
If you are feeling adventurous, you can follow the instructions posted here, or the more detailed ones posted by the dev/hacker himself at the PCWorld site here to get it up and running on your iPhone 3G.