Building apps for iPhone and Android

Hi guys

I'm making my way through:

Professional Flash Mobile Development: Creating Android and iPhone Applications - by Richard Wagner

I'm planning on making a single app for both iPhone and Android in Flash Pro

I understand that there is a different set up for each platform, but I was wondering if anyone here experienced in app making using Flash has any tips for developing for both platforms with minimal change to code - ie not having to change your main base code too much, and also, what you found you did need to change between platforms?

I'm especially interested in catering for different size screens. Ideally I'd like the app to stretch to fit whatever screen eg iPhone/iPad/large Android etc, but maintain it's aspect ratio. But I noticed in the demo apps in the book, you stipulate the viewport eg;

private const VIEWPORT_WIDTH:int = 320;

private const VIEWPORT_HEIGHT:int = 480;

So if you're stipulating the viewport size, how do you release one app to the apple marketplace that will suit both iPhones and iPads?

Thanks for your time and sharing your experience.