OK – sometimes you are so focused on one technology that you just forget another. A significantly simplified version of the MemberShipProvider i posted here:

public class WebConfigMembershipProvider : MembershipProvider

{

  public override void Initialize(string name, 
    System.Collections.Specialized.NameValueCollection config)

  {

    base.Initialize(name, config);

  }

  public override bool ValidateUser(string username, string password)

  {

    if (FormsAuthentication.Authenticate(username, password))

    {

      new AuthenticationSuccessEvent(username, this).Raise();

      return true;

    }

    else

    {

      new AuthenticationFailureEvent(username, this).Raise();

      return false;

    }

  }

}