If you are familiar with smart clients, you'll know that they allow you to use the full capabilities of the desktop including the screen real estate and connected devices as well as allowing a richer user experience.
We are continuing to build on our smart client story, and Client Application Services are part of this. Client Application Services allows you to use the authentication and role management of your web application (exposed via web services) inside your smart client application.
This saves considerable plumbing code that you'd have to write yourself and creates efficiencies for developers working to the Software + Services model.
In this screen cast I show the new extensions inside Visual Studio 2008 and walk through implementing it.