You can download the chat application that he shows testing from this expression blend blog post tutorial I did last month.
Jeff Wilcox (who developed the Silverlight unit test framework and harness) has a great blog post that talks about how to add a Silverlight Unit Test project to a solution here. In addition to shipping this unit test harness for Silverlight, we also shipped the source to ~2,000 unit tests built with it that provide automated coverage for the Silverlight control source that we also shipped under a permissive license (you can take the control source, modify it, run the unit tests to verify the behavior, then re-ship the controls however you want). This testing harness is cross browser and cross platform, and can be used to quickly run and verify automated unit tests: One of the important capabilities we shipped with the Beta1 release of Silverlight 2 was a unit test harness that enables you to perform both API-level and UI-level unit testing.