I guess json_data rather needs to be an jQuery.ajax call:
"json_data" : $.ajax({
    "dataType": "json",
    "url" : base_url+"/json",
    "data" : function (n) {
        return { id : n.attr ? n.attr("id") : 0 };
    },
    error: function(e,b,d){alert(d);}
})
                  Gumbo
                   2010-09-07 14:20:42