Around July 2013, Google released support for a BETA version of asynchronous AdSense code.
It went over very well, so well that it is now not only out of beta a few months later, it is also the default option when you create AdSense ads.
Now, when you create a new AdSense ad, the default option will give you the asynchronous route:
The new quick tool tip on this option reads:
The asynchronous ad code is a fully asynchronous version of the AdSense ad code that offers improved web latency and a better user experience for your site’s visitors. The advantage of the asynchronous code is that it's guaranteed never to get in the way of other parts of your web page loading. This means that users will be able to load the content of your pages, even if they’re having trouble loading the ads. Mobile users in particularly will enjoy less latency when loading web pages.
Forum discussion at WebmasterWorld.