Unit testing WPF GUI Framework
I prefer to use MVVM.
This moves most of the code to the ViewModel. The ViewModel can be tested using the testing tools in Visual Studio such as MS Test or NUnit.
For UI tests that remain (mostly custom controls) you could use a UI Automation test suite such as White on codeplex.
TestComplete is a great piece of software for automated UI testing - works with WPF. http://smartbear.com/products/qa-tools/automated-testing/
It will cost you though: at least $999.