This is very good for web sites that have ASP.NET Authentication (or plan to have) and have Silverlight apps.

Brad Abrams, from his blog:

"Here is what I plan to show:

1. Login\Logout
2. Save personalization settings
3. Enable custom UI based on a user's role (for example, manager or employee)
4. A custom log-in control to make the UI a bit cleaner
"

http://blogs.msdn.com/brada/archive/2008/05/03/accessing-the-asp-net-authentication-profile-and-role-service-in-silverlight.aspx

Source code available on the above post.