Depends what you're doing... if you're making a fairly low-level utility, then I'd carefully check out the available APIs for whatever you'll need to mess with

Depends what you're doing... if you're making a fairly low-level utility, then I'd carefully check out the available APIs for whatever you'll need to mess with (i.e. SMS). Otherwise, as far as supporting device differences like, say, multiple screen sizes and resolutions, it's just a matter of being careful about the APIs. Compared to developing for a single device, yes, you definitely need to put more thought and care into platform differences (though you can also target individual phones and OSs, if your app requires e.g. a physical keyboard for some reason rather than an on-screen one).