The Google Adsense team announced that you can now use rounded corners for your ads, giving you the option to choose between 3 types: Square corners (old style, default), Slightly rounded corners and Very rounded corners.

You can change this behavior from the AdSense Control Panel -> Choose Ad Format and Colors -> Corner Styles.

Hopefully this will make your ads a bit more attractive and improve the CTR (although in many cases it’s much better not to use any borders at all), anyway, the choice is up to you.