In this article Gill Cleeren is looking at cross-domain access to a service in Silverlight 4’s trusted applications.
As you may know, when accessing any service located in a domain different from the one in which the Silverlight application itself is hosted, Silverlight will check for a cross domain file being in place. Cross-domain policies prevent Silverlight applications to connect with services that are not in the same domain. However, a service can opt-in to be connected to if at the root of the domain, a cross-domain policy file has been deployed. Silverlight will check for its existence (the file should be named clientaccesspolicy.xml or crossdomain.xml, which is the cross-domain file of Flash) and if found, Silverlight will connect in a cross-domain manner.