With the current Silverlight releases available (1.0 and 1.1 aka 2.0) there is no way to automate the user interface. No support for Active Accessibility or UI Automation. But when signing up for the Beta of the Microsoft Download center, which uses Silverlight, I noticed this:
In my opinion this looks like the final release of Silverlight 2.0 will have Active Accessibility and/or UI Automation in place. This opens up the way for test automation. And I think that is a good thing!