According to http://www.zdnet.com/blog/perlow/the-long-kiss-goodbye-for-x86-desktop-windo... Microsoft is not letting developers use win32 on arm (although Microsoft is probably using it themselves for Office on arm). Instead, native apps have to use WinRT, a C++-based class library.
According to http://en.wikipedia.org/wiki/Windows_Runtime, the WinRT libraries are built on top of win32 internally.
If it takes off, and someone feels like supporting it in Wine, they would probably have to start by implementing the first bits in C like wine's msvcp support. Eventually, if and when llvm gets Visual C++ ABI support, maybe wine could start allowing C++ source code.
(FWIW, I see Charles Davis was working on that, http://code.google.com/p/google-summer-of-code-2010-llvm/updates/list http://lists.cs.uiuc.edu/pipermail/cfe-dev/2011-March/014305.html https://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/MicrosoftCXXABI.c... ).