There's been a bit of discussion about the difference between "desktop apps" and "Metro style apps" on Windows 8, particularly as it pertains to Windows RT (previously Windows on ARM), which currently isn't going to allow third party development of desktop apps. Part of the confusion comes from the fact that you can use native C++ code in a Metro style app.
Native C++ doesn't mean you can just recompile an existing C++ app written on top of Win32, though, and there are lots of misconceptions about what this means for converting existing apps to Metro.
I haven't actually written a Metro app, but I have looked a bit at what's involved out of curiosity. Looking at the documentation and the requirements, I'm of the opinion that most C++ apps will require a major rework to conform to Metro app requirements. This is based on the restrictions to Win32 API usage that will require invasive changes to either the C++ code itself or the frameworks that the C++ code is based on. Nevertheless, it's useful to actually look at the API lists and get an idea of where the trouble spots are, which is what I'm going to do here. Think of this as a what-if exercise, kind of what would happen if someone at work just asked you for a wild guess at what would need to change to retarget Metro.
Of course, in order to do this we'd need some guidelines, so we're going to assume: