OK, so it's not exactly "out of the box" but the most recent PRISM 4.0 Drop (drop 2) seems to contain something Silverlight developers have been begging for ... for quite some time.
It's buried within the "QuickStart" folder under the "BasicMVVM" project. This project provides guidance for the Model-View-ViewModel pattern and is worth a deep look. It may provide a basic framework for your future applications.
In that same folder, you'll find
BasicMVVM.Tests.AcceptanceTest. Fire this project up and take a look at it. It contains some helper classes and launchers to host the Silverlight automation piece. If you're impatient like me, build and fire it off. That is where you start to have fun!
Yes, that is the test panel within Visual Studio 2010, not the Silverlight Unit Testing Framework in a browser window. Even more interesting is watching the form appear on the screen, then automatically get filled out by the engine as the acceptance tests are run.
So what we have here is full UI automation for tests as well as VSTS integration.
This is obviously a huge jump for Silverlight ... I'm looking forward to digging in deeper to explore what's possible with this!