Here's a different kind of throttle. This one only allows a certain number (two) of outstanding requests; it queues subsequent requests until one of outstanding requests has returned.
A production version would need to check for error conditions too.

Results

var gQueue = [];
var gOutstanding = 0;
$.throttled = function(url, k) {
    function k2() {
        gOutstanding--;
        k.apply(this, arguments);
        if (gOutstanding < 2 && gQueue.length) {
            var entry = gQueue.shift();
            $.get(entry[0], entry[1]);
        }
    }
    if (gOutstanding < 2) {
        gOutstanding++;
        $.get(url, k2);
    } else
        gQueue.push([url, k2]);
};
for (var i = 0; i < 10; i++)
    $.throttled('services/sleep/2', log);