The patterns & practices team at Microsoft has released version 2 of the Composite Application Guidance (previously called Prism), now with support for WPF and Silverlight.

If you are looking for guidance in building modular WPF and Silverlight line-of-business applications, this should be your first stop. Included in this release:

• Composite Application Library
• Reference Implementation (Stock Traders application in WPF and Silverlight)
• Quick starts
• How-Tos
• Lots of documentation for everything you want to know about UI patterns  and client architectures

Looking at how to share source code between a Silverlight and WPF application? There is a tool that helps you do this. ProjectLinker links source files from project one and will make sure they stay synchronized in the second project.

Patterns & Practices Composite Applications Guidance on MSDN (links to downloads from here)
Community on CodePlex