In Silverlight, it can sometimes bet better to skip using the ServiceReferences.ClientConfig file and instead create your WCF clients from code, specifying the endpoint address. This was pretty simple in Silverlight 2 where all we had was basicHttpBinding. Silverlight 3, however, adds in the ability to use binary message encoding via the binaryMessageEncoding element in the server-side WCF config file – something that is on by default in the Silverlight Enabled WCF Service template.

Assuming you have a generated client (from a service reference) named “ExampleServiceClient”, here’s what the code looks like. The key entry is the BinaryMessageEncodingBindingElement. This may be obvious to folks who do tons of custom WCF work :)

private ExampleServiceClient BuildExampleServiceClient()
{
    BindingElementCollection elements = new BindingElementCollection();

    // order of entries in collection is significant: dumb.
    elements.Add(new BinaryMessageEncodingBindingElement());
    elements.Add(new HttpTransportBindingElement());

    CustomBinding binding = new CustomBinding(elements);

    // insert your favorite address resolution algorithm here
    EndpointAddress address = new EndpointAddress(
        new Uri("http://localhost:13519/Services/ExampleService.svc", UriKind.Absolute));

    ExampleServiceClient client = new ExampleServiceClient(binding, address);

    return client;
}

Thanks to Mike Taulty for pointing me in the right direction. I hope that saves you some trouble.