When building an app, it is often deployed in different environments (test, dev, prod), and therefore the endpoint addresses are changing. As the ServiceReferences.ClientCon
Have a look here:
http://weblogs.asp.net/srkirkland/archive/2009/10/13/common-web-config-transformations-with-visual-studio-2010.aspx
then here
http://www.funkymule.com/post/2010/03/08/XML-Transform-on-Silverlight-ClientConfig-Files.aspx
It uses the same principle behind the web.config transformations (ie, web.config is changed depending on what configuration you are compiling (ie release, debug) so that the serviceref.config is changed according to your whim upon compile time. works a charm