Loading JavaScript files asynchronously
I believe you don't need to use jquery deferred objects for this task. Just use script tags with sources - they will be downloaded in parallel from CDNs (try to use CDN, its always a good idea). The amount of parallel requests will be limited by browser restrictions.

Since your code just replaces default browser technique, for more sophisticated features (like dependent components, libraries that you really want to defer, because they are not used until some action on the page), use


http://jsload.net/
http://developer.yahoo.com/yui/get/
deferred objects as you did