I'm three days late blogging about this, but if you haven't seen Christian Cantrell's blog post of the same title you need to check it out. He shows an application he wrote that runs on five screens including Mac, Windows, Linux, iPhone, and Android. I'd argue it's more like seven screens given the different device operating systems. Christian says he wrote the application code once and different wrappers for each operating system. I know this is supposed to be possible given the Flash Player runs on Windows, Mac, and Linux and Flash Professional CS5 can compile a Flash app to native iPhone code. This would get his app onto iPhone, iPod Touch and iPad. But Christian says his app is able to run on all the devices because it's built with AIR. Ok, cool. I want to see /how/ though.
Christian promised to release his code as open source so inquisitive developers like me can learn more about this.