Hacker David "Planetbeing" Wong has posted a video of Google's Linux-based Android OS running via OpeniBoot on the iPhone. Obviously, it's way off of Apple's reservation, but as you can see in the video above, it works very well -- you can browse the web, play media, and even send and receive calls and messages. If you want to give it a shot (and of course, with all iPhone hacks, the usual disclaimers apply -- you might brick your iPhone or worse, cause Steve Jobs a headache), the files are all available for download from his site.
Currently he's only got it running on the 1st generation iPhone, but he says that it should eventually be able to work with later versions. His goal, he says, is to eventually provide iPhone users with a supplementary or even an alternative OS, but as with most of these hacks, it seems like more of a novelty rather than a useful implementation. Still, if you've been itching to free the OS bonds of your 1st gen iPhone, now's your chance to try and slip free of the chains.