The Silverlight Unit Test Framework ships officially inside of the Silverlight Toolkit, since the team has a dependency on it for thousands and thousands of unit tests.
I recently did hear a few complaints that the Silverlight Unit Test Framework site on the MSDN Code Gallery is a little out of date. I hear you, and am going to try to refresh the wiki and documentation a little bit.
In the meantime, I’ve updated the site with the latest Silverlight 2 Unit Test Framework binaries for download. Here’s a direct link to the download. I don’t have any updated bits available for the Silverlight 3 Beta at this time.
What’s new in the July 2009 bits of the Silverlight Unit Test Framework?
more here