Savas and I had talked about this idea of using Silverlight-based cycle stealing and wondered how well it would work. It's good to see this article on CodeProject about Legion: Build your own virtual super computer with Silverlight by Daniel Vaughan.
Legion is a Grid Computing framework that uses the Silverlight CLR to execute user definable tasks. Legion uses an ASP.NET application and web services to download tasks, upload result data, and provide grid-wide thread-safe operations for web clients or agents. Multiple tasks can be hosted at once, with Legion managing the delegation of tasks to agents. Client performance metrics, such as bandwidth and processor speed, may be used to tailor jobs for clients. Legion provides a management service and WPF application that is used to monitor the Legion grid.
I have deployed Legion to a demonstration server here so you can see it in action.