Last couple years I have been using SWIG to make pure (unmanaged) C++ DLL's available in WPF applications. It works great, no problems at all, and this is for big applications such as avionics maintenance trainer.
SWIG is great because with one set of interface files (which you write based on your .h header files, so quite easy), you can generate all the scaffolding code to export your C++ DLL to multiple languages such as C#, Python, Lua, Java. SWIG generates the code you need if you want to extend a C++ class from C# or Lua, it handles passing exceptions across languages (so a Lua script could throw an exception that trickle through the C++ layer and get caught at the C# level, for instance), etc. We never have to touch PInvoke (SWIG does it all). It's amazing.
This way, the same SWIG interface allowed us to use WPF (C#, with .NET 4) for the GUI, a native C++ backend (via SWIG) for some existing libraries we had to integrate with, and embed a Lua interpreter to support scripting of the application (via SWIG, same .i interface files, and lua-icxx on sourceforge to simplify interfacing with Lua C API interpreter stack).
I'm wondering how Qt/QML compares to WPF/XAML now, 2 years after this thread started. And I see there is now Qt3D to add builtin 3D as has been available in WPF for a while now (supports 3D canvas natively, without OpenGL or DirectX).
