TDD type methodology for WPF / Silverlight
The silverlight toolkit provides all the unit test and the source code...on http://silverlight.codeplex.com/
For UI testing I think some folks use the Silverlight Accessibility apihttp://msdn.microsoft.com/en-us/library/cc707824(VS.95).aspx....
And there are also tools for UI testing, too
MSDN Magazine article on UI-Testinghttp://msdn.microsoft.com/en-us/magazine/dd483216.aspx
Testing using WPF UI automationhttp://blogs.msdn.com/b/llobo/archive/2007/09/06/testing-using-wpf-ui-automation.aspx
It is surely not an easy and obvious topic and it always depends on the business case...
And for TDD in general look at this book: The art of Unit Testing...