I'm having the same problem and I can't seem to solve it, even through the link you provided.
Starting a blank Silverlight application, it's easy as pie to use the ServiceReference file, but with a Silverlight Viewer Application there's no obvious soltution. Nothing I've tried so far works, except bypassing the file by setting up the bindings in code behind, but that is not an option unfortunately since I need different configurations for different environments.
Is there any additional help to get? Not being able to use this is extremely time consuming.
EDIT: My ServiceReference.ClientConfig is in the .xap, but it is not accessible when. Debugging results in the error "Cannot find 'ServiceReferences.ClientConfig' in the .xap application package.".