Are you sure you're not describing cross-platform with Windows Phone instead of Windows 8?
I thought for Windows 8, Microsoft is pushing WinRT as the cross-platform solution, and that "binaries" crossing from x86/x64 to arm will work as-is for javascript, as-is (if pure) for CLR, and with no modifications but requiring recompilation for native (C/C++) WinRT apps.
If you're writing in C/C++ rather than C#, then you're off the CLR ranch, so to speak. It's like saying Java isn't portable because a program links to custom libraries written in C.
That said, rumor has it that the next version of Windows Phone will not be Silverlight (though XAML and Silverlight are fairly similar), but based on WinRT.
That's a good thing because it will allow Windows Phone Apps to better integrate Javascript.
I thought for Windows 8, Microsoft is pushing WinRT as the cross-platform solution, and that "binaries" crossing from x86/x64 to arm will work as-is for javascript, as-is (if pure) for CLR, and with no modifications but requiring recompilation for native (C/C++) WinRT apps.