What's needed to succesfully make a crossdomain call from Silverlight?
If I understand your question correctly you would need to have a clientaccesspolicy.xml file in the domain web root of the server that you wish to call (ie www.example.com/clientaccesspolicy.xml) that defines that it is ok for services from other domains to call services on that domain.
Read the How to Make a Service Available Across Domain Boundaries MSDN article for more detailed information.
See Jon Galloway's blog post on this as well
http://weblogs.asp.net/jgalloway/archive/2008/12/12/silverlight-crossdomain-access-workarounds.aspx
Intellisense helper file and walk-through: http://silverlight.net/learn/learnvideo.aspx?video=47174
Maybe also check out JSONP http://www.west-wind.com/weblog/posts/107136.aspx for example this is how you can get Twitter updates in JavaScript on the client side even though Twitter is on a different domain than you web page.