Windows 8, C++ and Metro GUI samples?
Sample code in C++ and other languages is at http://code.msdn.microsoft.com/windowsapps. You can take a look at how it's done.
If you want to call C++ code the easiest way will be to use C++ with Component Extensions. This is just plain C++ that compiles to native code however it has a few extensions (reminiscent of C++/CLI) that let you use the WinRT COM components without worrying quite so much about the COM plumbing.
With C++ and WinRT you can actually use XAML like the managed languages to define your user interface. It's pretty neat, see the documentation here:
I haven't looked into it but you may still be able to use P/Invoke or COM interop in the managed languages to call C++ code for a Metro style app however this is unconfirmed. Obviously a desktop app can do all the things it normally would.
You can use C++ code to write metro style applications. You can also write applications in Javascript/HTML/CSS and call APIs that you write in C++ or C#/VB from those JavaScript applications.