Silverlight testing: Watin vs Selenium comparison
I can't talk to Watin or Selenium with Silverlight, but I have played with White, which is a layer on top of the MS Automation Framework, and I have liked what I have seen thus far:
I should also add that we defer the majority of our UI behavior testing to unit tests using the MVVM pattern. It doesn't test EVERYTHING, but it gets us 95% of the way there.
Functional testing frameworks like Watin/Selenium/White should be used in conjunction to unit tests.
What about the Silverlight Unit Test Framework included in the Silverlight Toolkit?
Here's Scott Gu's intro
Not sure if this suits your needs, but I always like to check out the solution from the source itself!
I've not had the need to use it myself, but I have noticed that there is a Silverlight Selenium project 'Silvernium', which might be of interest to you. http://code.google.com/p/silverlight-selenium/