This blog post of Karl Shifflett is a proof-of-concept for Non-Linear Navigation in Silverlight 4 Navigation Applications.
I first heard about Non-Linear Navigation in this dnrTV video that Billy Hollis did. While on a cruise to Alaska I wrote the BBQ Shack application, which is a WPF implementation of Non-Linear Navigation.
Non-Linear Navigation is one way for Silverlight line of business applications to provide users with a desktop application like experience. This would include both in-browser and out-of-browser applications.