If you've ever struggled to try and come up with a strategy for doing automated unit testing or TDD with AJAX applications, I think you'll find Silverlight provides some much nicer test options.  Using Visual Studio you can also separate your tests into a separate project in your solution, and you do not need to embed the tests within your Silverlight application in order for them to run.

In addition to supporting the above unit test harness and framework, we are also going to support UI automation APIs with the final release of Silverlight 2.  These will enable accessibility scenarios (allowing screen readers to work with Silverlight and enable Section 508 compliance of Silverlight applications).  These UI automation APIs will also enable UI testing scenarios where you can build end to end browser UI automation that simulates real mouse and keyboard interactions and enables automated end to end experience testing.  The combination should enable you to build much more solid and maintainable RIA solutions.

Hope this helps,

Scott

P.S. For more tutorial posts and links on Silverlight 2, check out my new "Silverlight Tips, Tricks, Tutorials and Links" page.