Although we don't have an official Moq for Silverlight yet. You can go to the email list and pull the patch I submitted to get an early version up and running.
I have, in the queue of blog posts, a Silverlight AutoMocker leveraging the new Microsoft Unity container for Silverlight.
I'll wait till we get a full Silverlight version of Moq before posting.
The existing Moq automocker for the full .NET framework, over at Moq-contrib, is leveraging the Autofac contianer, and when I threw my Silverlight version together Autofac hadn't been ported (yet), however just read a blog today that said it has an early port available.