In this post you'll learn how to host a Silverlight application (aka a .xap) on a server, making it available to be called from other servers.
I just had to do this and figure I'll document the steps. It isn't rocket science, but here's the story.
First, you need to place a clientaccesspolicy.xml or a crossdomain.xml file at the root of your server. Read this MSDN documentation for more.