$.cacheFactory

gistfile1.js

$.cachedGetScript = $.cacheFactory(function(dfd, url) {
    $.getScript(url).then(dfd.resolve, dfd.reject);
});

jquery.cachefactory.js

(function ($) {
    $.cacheFactory = function (f) {
        var cache = {};
        return function (key, cb) {
            if (!cache[key]) {
                cache[key] = $.Deferred(function (dfd) {
                    f(dfd, key);
                }).promise();
            }
            return cache[key].done(cb);
        };
    };
}(jQuery));

example here: http://jsfiddle.net/abernier/nu8e2/