<script src="code.js" async></script>
<script src="code.js" async="async"></script>
The same implementation is used for "defer".
To see async in action you need a modern browser. Firefox 3.6 will do the trick.
I have put together a couple of examples on using the async attribute and decided it would be best to demonstrate them in video. The following video shows the script running in regular, blocking mode, and then with async turned on. I am using Firebug to dramatize the difference.
Sorry for the bad audio quality.
Some things to note:
This change will most likely be beneficial for things like ads or widgets that have self-contained scripts that run on the page. That way the ad and widget code won't block the rest of the site if it is slow to load.