It's very rare that the situation isn't really "develop once, debug everywhere." You might as well promise me a perfect optimizing compiler while we're at it.
And even then, is the GUI actually appropriate for all of your deploy targets? Usually not.
Have we had that confirmed? I was hearing that compiling a WinRT application for phone and desktop was going to take tweaking and recompilation.
I do know that design tools like blend support multiple layout views for different screen shapes and orientations. I can't see how this would be different. Underneath if you write .Net code it'll run on both.