Windows 8 and Windows RT apps are programmed using WinRT in HTML5/JS, C++, C#
Windows Phone 8 apps are programming using WinRTP in C++, C#
WinRTP is a subset or modified version of WinRT. There won't be direct app compatibility, but the majority of the code can be shared between the two flavors of the app. You don't really want to run phone apps on a tablet and vice versa anyway, the change in screen size and interaction necessitates a change in UI.
I would think though, with the newer apps being able to support 16:9 scaling that it would be something that would be great if it were portable across all three platforms. I use a 27" flat screen for my desktop, would truly love to have a large screen smartphone(big hands/fingers) since I rely on sliders for the majority of my input since going stylus-less with these newer items and would think that the Surface would also be a 16:9.
Yes, there may be some issues with the newer lower end devices since they are not in the 16:9 format, but I would assume there is something in the coding requirements or processing by the OS itself that would reformat it to fit on the other devices.
Just as a simple example:
Wouldn't it be nice if I could play Angry Birds on my computer. Then, later while sitting in the living room, pick up my Surface(Intel or ARM) and carry on where I left off. Maybe later, while sitting in the Dr's office waiting on my appointment, pick up my phone and carry on from there. After I get home later in the evening relaxing by picking back up on my large screen desktop.
Just a thought. Not just for game play but pretty much any application/program you would use. Office, Accounting, Email, really pretty much anything you could think of that you would use. With cloud services, I could see it as a place to store the data and then the applications would automatically and seamlessly pull that data no matter what system you are on.