Check out the Microsoft Case Study I recently was able to participate in a for a .NET 4 / Silverlight 4 application I worked on.
One aspect of working with Silverlight is that all service calls are asynchronous. This creates a necessity of designing your code to allow for notification of when service calls complete. Those callbacks are usually private methods, or even inline delegates and lambdas.
Recently we upgraded our Silverlight 3 application to Silverlight 4, WCF RIA Services, and ASP.NET MVC 2. For the most part, the upgrade went very smoothly. However RIA Services just quit working. We were getting “Load operation failed for query ‘GetAbcd’.