Accessing external data feeds with Silverlight 2.0
Along with this difference is the ability to access data. ASP.NET applications can use the HttpWebRequest or call a web service not located on the same server. Since Silverlight runs on the client, it is a potential security risk to access data from a server where the XAP did not originate. SO, the introduction (it’s a standard practice for Flash) of the client access policy. Briefly, create a file named clientaccesspolicy.xml with the xml content below, and your service/data is ready to be consumed by Silverlight.