I've read many posts on this and they all suggest what I am already doing:

jQuery javascript:

var responseText = $.ajax(
{
   type: "POST",
   contentType: "application/json",
   url: "WebServices/MyServices.asmx/GetJSON",
   data: "{}",
   success: function(msg)
   {
      if(msg.d)
      {
         var myObj = $.parseJSON(msg.d);
         //do stuff with myObj
      }
   }
});

Note, I am explicitly specifying both POST and contentType application/json.

In the asmx:

[WebMethod]
[ScriptMethod(UseHttpGet=false, ResponseFormat=ResponseFormat.Json)]
public string GetJSON()
{
     return "{\"testData\":\"Whatever JSON Data I want\"}";
}

Note, I am explicitly specifiying ResponseFormat correctly and indicating that it will not use GET.

When I run it within Visual Studio, it works as expected, returning JSON-formatted data with the annoying "d:" attribute

{"d":"{"testData":"Whatever JSON data I want"}"}

and my javascript works just fine accessing the actual JSON data.  

When I deploy this to an II7 server, however, it comes back as xml:

<?xml version="1.0" encoding="utf-8"?>
<string xmnls="http://myDomain.org/">{"testData":"Whatever JSON Data I want"}</string>

Seems like a configuration issue or something.