Re: Plus, Apple are very busy alienating developers...
It was just a quick 30 second back-of-a-<textarea> sketch. I know it's a little more complex than that, but for particularly games that run full-screen and don't use the built-in UI widgets, it's not much more complex. The idea is that you never use px values. Use an arbitrary unit of your own definition (let's say millimetres, em-spaces, linguines or dragonlengths) and then define how many pixels that means later, when you can get the current device's screen height and width.
As for stuff that uses the UI widgets, well I think someone already mentioned Apple having a smart re-flow tool. As for Android, the page at http://developer.android.com/guide/practices/screens_support.html shows this is a done deal and something that's quite comprehensively explained. Basically the answer to how you support many screen sizes is "you don't have to, it scales itself, but here's some methods to make it look even nicer."
Also hm, this thing has stopped understanding hypertext. <i>Well foo.</i>