A WebmasterWorld thread reports finding the destination URL change dynamically as you mouse over a Google AdSense ad.
For example, as I mouse over an AdSense ad, when I look in the status bar at the bottom, I can see the end of the URL change from &nm=01 to &nm=02 to &nm=03 and so on.
Here are screen captures:
I have never seen this myself, but that doesn't mean it is incredibly new. If you like to see it in action, I will place an AdSense ad below (please do not click on the ad, just mouse over it and look at the destination URL).
Very interesting find.
Forum discussion at WebmasterWorld.