@Ashley, you should implement a template system like Adobe Flash does.
The template only adjusts the stages size for the user but it's quite a nice little feature.
So you could have iphone, android, ipad ect.
I know the supporting multiple screen sizes is the way you suggest to go, but when I make a game for different devices I change a few things, especially when it comes to interfacing on the iPad.
But it's mainly to do with DPI, if im developing for the retina display I use a dpi of 326 for my graphics, for older devices 163. So supporting multiple screen sizes is not the smartest thing to do for mobile devices.