Mike Taulty has published an excellent series of screencasts illustrating some of the ideas found in "Prism" or the "Composite Application Guidance" from the Patterns and Practices team that can be used to build Silverlight applications in a way that lends itself to testability and modularity.

• Part 1: Taking Sketched Code Towards Unity
• Part 2: Dependency Injection with Unity
• Part 3: Modularity with Prism
• Part 4: The Unity Bootstrapper

• Part 5: Moving to a Modular Silverlight Project
• Part 6: Shells, Regions, Views
• Part 7: Commands
• Part 8: Loosely Coupled Events with Event Aggregation
• Part 9: Sharing Data via Region Contexts

• Part 10: A Larger Example: "Email Client"