<p>What's needed to succesfully make a crossdomain call from Silverlight?</p>

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.