Credit:
On his blog, Wang has posted the source code and explains how the port works. Right now it's running on a dual-booting original iPhone, but the goal is to get Android working on the 3G and 3GS as well.
Check out the video demonstration below. It may not be the world's fastest Android phone, but it looks fairly responsive overall. Keep in mind that though running third-party OSes on the iPhone is technically feasible, if you want to rig this up you'll have to do so at your own risk.