JSONP Callback Method Is Not Defined

I'm trying to get a jsonp callback working using jquery within a greasemonkey script. Here's my jquery:

$.ajax({
[URL],
data: { authkey: "temphash" },
type: "get",
dataType: "json",
cache: false,
success: function(data) {
console.log(data);
}
});

in my webservice (asp.net) I'm returning the response with a content type of application/javascript. The response the server is actually sending back is: jsonp1276109314602({"message":"I'm getting tired of this not working"}) The jsonp1276109314602 method name is being randomly generated by jquery, and I'm grabbing it with Request.QueryString["callback"] However my success function is never called and the firebug console gives me an error saying jsonp1276109314602 is not defined. What am I doing wrong?

NOTE: I'm making this call from a greasemonkey script on a craigslist page. It's a cross-domain request, but I can see the request is actually making it to the server and returning a good response, but for whatever reason the registered callback that jquery creates appears to not exist when the response comes back. It works fine if I run the script in the firebug console from the craigslist page, but not when it's run from the greasemonkey script.