I've put together a solution I use as a template for creating modular apps using Prism 4.0 with Silverlight 4.0.
I have it posted on my website. Please post any comments there if possible.
It should be all you need to get you started with a real world SL 4, modular, LOB application. It uses Telerik's RibbonBar as the apps menu.
Prism team, I'd love to get something like this added samples. If you have suggestions about generic items for a base template project please let me know.