In this article published on MSDN, you can find how to deal with those pesky WCF CommunicationException “NotFound” errors in Silverlight.
Silverlight version 4 enables support for the Windows Communication Foundation (WCF) SOAP fault programming model, which allows the service to communicate error conditions to the client. In previous versions of Silverlight, if an error occurred on the service, it would register as an HTTP 500 response code and details about the error would not be accessible to the Silverlight client.