Then you would typically define a method that looks like the following that is accessible via a RESful call (if using WCF) - note: the UriTemplate here is what is important:
[OperationContract, WebGet(UriTemplate = "/clientaccesspolicy.xml")]Executing any method from within a desktop Silverlight app whether in-browser or out-of-browser (SL 3 feature) will call that method above implicitly by the SL HTTP stack.
When calling any web method from a Windows phone Silverlight application, the 'GetClientAccessPolicy' is not called so there is (currently no need for it if your clients are from WP7 apps).
I'm not sure if this will change when the WP7 tools RTM on the 16th September - I will be the first to try!
My colleague Anthony Cooper found this out, so thanks to him.