If you are starting to get into integrating web services with Silverlight, you'll notice that you have to have a cross domain policy file in place on the target server, that is to say, the server hosting the service you want to implement. There are some public web services (Flickr, YouTube, Digg, etc.) that already have these files in place for Flash, but implement in a slightly different way.
When calling a cross-domain service, Silverlight will check for the existence of clientaccesspolicy.xml first. This is the format defined by Silverlight and provides a pretty flexible way to define who can access...