As you might noticed in the previous post, i’m working on a implementation of a custom MembershipProvider.
Well here is another useful link that shows how you can easily expose the AuthenticationService for your Silverlight application as a WCF Service:
Accessing the ASP.NET Authentication, Profile and Role Service in Silverlight
I will probably have some other posts coming up about using the MembershipProvider in Silverlight and ASP.Net, so keep an eye out.