It seems the Model-View-ViewModel (MVVM) is the preferred pattern at present for implementing cleanly separated SL apps (& WPF client apps). Aims to increase testability and leverage data-binding in XAML to implement the View part of the pattern. The Model is responsible for accessing data (e.g. WCF service, REST, etc) and passing it to the ViewModel (which holds state and provides operations to the View)- the View then binds to objects in the ViewModel class. A view has a reference to a Model interface rather than a concrete implementation, this allows testing of the Model in isolation. We could implement the ViewModel as an interface or for testing purpose just build a stub Model and pass that the the ViewModel (going the whole hog you could use dependency injection such as Unity for Silverlight or Ninject to wire the parts together).
More info on MVVM: