When develping a large UI project, Composite Application Guidance is a great framework for breaking large pieces into smaller chuncks using model-view-presenter pattern so that multiple developers can work on it at the same time. However, the current Composite Application Guidance for WPF, Prism 1.0, does not support Silverlight. Luckily, Prism 2.0 is on the work so hopefully we will see Composite UI for Silverlight soon.