http://blogs.msdn.com/b/alexj/archive/2009/11/13/tip-43-how-to-authenticate-against-a-data-service.aspxProblem: When writing code against a Data Service, like say SharePoint , the client application must provide a valid set of credentials, or you will see a dreaded “401 Unathorized” response. For Silverlight applications hosted on the same site asen-USTelligent Evolution Platform Developer Build (Build: 5.6.50428.7875)