Well, now I'm thoroughly confused!
I've got references to Moq.Silverlight.dll, as well as Castle.Core-Silverlight and Castle.DynamicProxy-Silverlight.
I've tried adding in the Silverlight-less versions of the dlls, and I get an error saying that I can't add it as it wasn't built against the Silverlight runtime.
I've got version 4.0.10827.0 of both Moq dlls.
If I run the project via VS2010, then it runs and runs all the tests. I do get some tests failing.
However, if I look at Moq.Silverlight.dll in Reflector, and it has Castle.Core.dll in the list of references, and if I click on it, it is unable to find the dll, so there's definitely something screwy going on with my setup!
Cheers for looking into it, it must be something we've done here. I'll let you know if we work it out!