Read original post by Roy Dallal at Dll Shepherd.Net
In the past month our team started a new project and after many discussions of how to build it right, decided to use the Prism framework (you an read more on Prism here). I was put in charge of providing a service (as in a Silverlight utility class) for Authentication and Authorization using Windows Authentication and Active Directory groups.
The way we handled that in our previous project was using the built in services in RIA services (the only use we had for RIA Services).