Josh Smith has a nice CodeProject article that discusses the Model-View-Controller Design Pattern with respect to creating modular WPF Applications. He then dives into unit testing the model and controller and introducing mock objects.
Per the article:
"This article has been bouncing around in my head for a long time, just itching to get out. Initially I wanted to focus solely on how to implement the Model-View-Controller pattern in WPF, but eventually I realized that there is already some great material on the Web about that topic. This article introduces how to implement MVC with WPF, but that is merely a prerequisite for explaining how the application’s unit tests work. Toward the end of this article, I introduce the Model-View-ViewModel pattern that some very smart folks at Microsoft invented specifically for WPF. MVVM is a powerful and useful alternative to MVC when designing a WPF application."
Check out Josh's article here.