It's been working for me with the new versions of jQuery. I think the problem is you're using a POST request, which isn't allowed with JSONP:
You can't POST using JSONP...it simply doesn't work that way, it creates a element to fetch data...which has to be a GET request. There's not much you can do besides posting to your own domain as a proxy which posts to the other...but user's not going to be able to do this directly and see a response though.
From How to use type: "POST" in jsonp ajax call
Your call should look like this:
$.ajax({
url : 'http://domain.local/api/3/authentication/get-token',
type : 'GET',
dataType : 'jsonp',
data : 'username=user&secret=pass',
success : function(data) {
console.log(data);
}
});
If the above doesn't work, try looking at the parameters sent using Firebug. jQuery should send a "callback" parameter with a value like "jQuery1710013558088336139917_1344030860953."
Also look at the response from the server. The server should be returning be returning data which looks like the following:
jQuery1710013558088336139917_1344030860953({"data":"goes here"})