When Google introduced the arrows to the AdSense ads, they came with a colored square background button with an arrow overlaid on the square. Then a year ago, Google went from the square background to a circle background look for the AdSense ads.

Here is the before and after back then:

Now Google seems to be testing the square arrow blocks again. Here is a picture from a publisher in a WebmasterWorld thread:

It is not unusual for Google to bounce around with ad formats in order to change up the ad styles enough to keep the click through rate somewhat up.

Forum discussion at WebmasterWorld.