I've been playing around with Silverlight 2 beta in Visual Studio 2008. I tried to create a Test project in my Silverlight solution and add a Silverlight project as a reference to the Test project. No go! Complains about Silverlight assemblies can only be referenced by other Silverlight assemblies. This sucks...I wanted to drive a lot of my design of non-visual classes in the Silverlight project using tests. I assume that NUnit testing won't work either. Could be very interesting developing for the CoreCLR.