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:

Forum discussion at WebmasterWorld.