In Google's pursuit to make the web faster and leave the slow behind, Google Analytics announced the ability to add Site Speed metrics to your Google Analytics reports.
All you need to do is add the line of your asynchronous code, _gaq.push(['_trackPageLoadTime']);
The day after that is in, you will be able to start viewing the load time of your site's visitors. The feature is found in the new Google Analytics under Content and then Site Speed.
Here is a picture of how slow this site is:
Compare that to how slow I am on Google Webmaster Tools and Page Speed - I clearly need to do something about it.
Forum discussion at WebmasterWorld.