Silverlight based XMPP clients need to retrieve a clientaccesspolicy.xml file similar to how a flash based xmpp client needs to retrieve a crossdomain.xml file. A silverlight client can use a crossdomain file, but will always ask for a clientaccesspolicy file first resulting in a wasted HTTP transaction if the clientaccesspolicy file is not present.

See http://msdn.microsoft.com/en-us/library/cc197955(v=vs.95).aspx for more details.