ok apparently almost all tutorial on the internet use these lines as clientaccesspolicy.xml put in like this
<?xml version="1.0" encoding="utf-8"?>
<access-policy>
<cross-domain-access>
<policy>
<allow-from http-request-headers="SOAPAction">
<domain uri="*"/>
</allow-from>
<grant-to>
<resource path="/" include-subpaths="true"/>
</grant-to>
</policy>
</cross-domain-access>
</access-policy>
this line cause a major issue on this line since silverlight 4
<allow-from http-request-headers="SOAPAction">
we have to change the http-request-headers="*"> in order to make the web service works.