Unlike in the desktop framework, Silverlight doesn't have a good extensibility point for replacing the serializer which is used to serialize / deserialize operation parameters (since the class DataContractSerializerOperationBehavior is not public in SL4). I wrote a post about how to replace the default serializer using the existing extensibility points in SL4 (endpoint behavior, message formatters). Check it out at http://blogs.msdn.com/b/carlosfigueira/archive/2011/05/24/wcf-extensibility-custom-serialization-in-silverlight.aspx!