today i had to make silverlight call WCF service so i made a normal wcf service and try to consume it with silverlight and it didn't work ,so i start the debug jury and it lead to no where ,then i serve the forums , blogs ,and documentation but no solution
you can check on silverlight forum.
http://silverlight.net/forums/p/1119/2597.aspx
then i suspected that maybe silverlight client access policy is a must to connect to web service even in debug mode  so i added  clientaccesspolicy.xml  to my wcf  service and it works fine now

the code for clientaccesspolicy.xml :
<?xml version="1.0" encoding="utf-8"?>
<access-policy>
  <cross-domain-access>
    <policy>
      <allow-from>
        <domain uri="*"/>
      </allow-from>
      <grant-to>
        <resource path="/" include-subpaths="true"/>
      </grant-to>
    </policy>
  </cross-domain-access>
</access-policy>