The extensibility interfaces (IClientMessageFormatter, IClientMessageInspector, IParameterInspector) added in Silverlight 4 were a great addition and enabled many scenarios that were, if not impossible, quite hard to accomplish before. But for applications written for SL3 (and Windows Phone 7), those extensibility points aren’t available, so doing things such as inspecting messages, adding headers, customizing serialization, etc. was practically impossible. Yavor had posted before about using a custom channel to simulate a message inspector to workaround a bug in the SL communication with sharepoint, so I decided to take his idea and expand it to include the remaining interfaces (and also add support for correlation state in his inspector). Check it out at http://blogs.msdn.com/b/carlosfigueira/archive/2011/06/21/wcf-extensibility-extensibility-in-windows-phone-and-silverlight-3.aspx!