hey,
I write about similar problem but this is other... When I try to consume WCF service in Silverlight I get in Firebug 404 not found error in crossdomain.xml file. But when I add to IIS port by which I use to communicate with service for example http://localhost:11598/Service1.svc and restart IIS of course , I don't get it error - crossdomain file was ok , but my Service get 404 not found error :/