We've seen various Android hacks before, but this is more of a pact with demons than a bit of software trickery. A young coder named David Wong has managed to get Android dual-booting with the regular OS on an iPhone 2G.
In the video above, you can see the process - involving the loading of a Linux kernel using a bootloader. If you just want to see it in action, skip to about 3m05s in the video. Wong demonstrates connecting to a Wi-Fi network, browsing the web, sending an SMS and making a call.
He bemoans the lack of buttons on the iPhone, saying that he's been forced to repurpose the volume rocker as the home and call buttons. He reckons that it'd be relatively easy to upgrade the hack to the iPhone 3G, but that the 3GS would take more work - presumably due to its different hardware.
Thanks to SoapyWeasel for the tip.