The Scribble application is back in Visual Studio 2008! Ok, it's not WPF, but at least it exists (http://msdn2.microsoft.com/en-us/library/06s5xsah(VS.80).aspx). It even has a tutorial on extending the application to include a plug-in model (http://msdn2.microsoft.com/en-us/library/11z7687s(VS.80).aspx).

It probably pays to start with the WinForms version of the example. Make sure you change the target CPU to Win32 (unless you're on an Itanium) so you'll be able to compile and debug. From there, you can wander through the code and figure out why all of the UI is created via code rather than the clever UI form designer that has been so nicely integrated into VS Orcas (2008). If you figure it out, let me know. Reading UI and UI layout code isn't my idea of a good time.

After the WinForms example, take a look at the MFC sample. It's a little involved, but definitely a nice example of integrating native C++/MFC with managed libraries using managed C++ proxy classes to expose managed interfaces to the plug-in libraries.