On the page you provide the json you have to accept a para callback and use this parameter to generate the function name. look what's happen when you call jsfiddle callback with an other name:
http://jsfiddle.net/echo/jsonp/?test=some+html+content&callback=poney
give:
poney({"test": "some html content"});
You have more information here: jQueryDoc
If the URL includes the string "callback=?" (or similar, as defined by the server-side API), the request is treated as JSONP instead. See the discussion of the jsonp data type in $.ajax() for more details.