All of the samples I've found that show how to build a JSONP service using the Windows Communication Foundation have the thing hosted in IIS.  IIS is literally the last place I want my Windows Communication Foundation services hosted ... especially because I'll only want them in IIS once I've debugged them, and debugging outside of IIS is much more efficient.  Here is a console version of such a service:

 

using System;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Description;
using System.ServiceModel.Web;

namespace Server
{
    [DataContract]
    public class Some
    {
        [DataMember]
        public string What;

        [DataMember]
        public string Why;
    }

    [ServiceContract]
    [JavascriptCallbackBehavior(UrlParameterName="method")]
    class MyService
    {
        [OperationContract]
        [WebGet(ResponseFormat = WebMessageFormat.Json)]
        public Some GetSome(string input)
        {
            return new Some() { What = "A Shotgun", Why = "Needs an Outcome" };
        }
    }

    class Program
    {
        const string BaseAddress = "http://localhost:8888/MyService/";
        const string EndpointAddress = "MyEndpoint";

        static void Main(string[] args)
        {
           
            using(ServiceHostBase serviceHost = new ServiceHost(typeof(MyService),new Uri[]{new Uri(Program.BaseAddress)}))
            {
                WebHttpBinding binding = new WebHttpBinding();
                binding.CrossDomainScriptAccessEnabled = true;
                ServiceEndpoint endpoint = serviceHost.AddServiceEndpoint("Server.MyService", binding, Program.EndpointAddress);
                WebHttpBehavior behavior = new WebHttpBehavior();
                WebScriptEnablingBehavior scripting = new WebScriptEnablingBehavior();
                endpoint.Behaviors.Add(behavior);
                endpoint.Behaviors.Add(scripting);
                serviceHost.Open();
                Console.WriteLine("Press <ENTER> to terminate this program.");
                Console.ReadLine();
            }
        }
    }
}

 

And here is the HTML page that you can open in the current version of any major browser to see the output from the service:

 

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>Untitled Page</title>
    <script src="http://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.5.1.js" type="text/javascript"></script>
    <script type="text/javascript">
        function callback(data) {
            alert("Like a " + data.What + " " + data.Why);
        }
        $(document).ready(
            function () {
                $("<p>Hello, I am Dynamic Text!</p>").appendTo("body");
                $("<p>Attempt</p>").appendTo("body");
                $.getJSON("http://localhost:8888/MyService/MyEndpoint/GetSome?method=?", callback);
                $("<p>Done</p>").appendTo("body");
            }
        );
    </script>
</head>
<body>

</body>

</html>