It doesn't appear there is any VS IDE integrated support for running SL unit tests but there is a browser-based unit testrunner, 

 

To get the correct templates follow this straightforward guide:

 

 http://blogs.vertigo.com/personal/aanttila/Blog/archive/2008/12/29/getting-started-with-silverlight-unit-testing.aspx

 

Then just create your unit test project(s) and write tests using the same attributes as [TestClass] [TestMethod] etc.

 

To run your SL tests (they don't show in VSs Test View window) set you SL unit test project as Startup and run, a browser opens and runs the tests, detailed instructions at http://www.jeff.wilcox.name/2008/03/silverlight2-unit-testing/#option2