AdSense add Corner Styling Options to Ad Unit Setup
If you log into AdSense today and create a new ad unit you’ll be given the choice of creating ad units with rounded corners.
You’ll now have three choices:
Access the different options through the ‘Setup Tab’ of AdSense but keep in mind that if you have a blended approach to creating ads (your borders being the same color as the background of the ad and your blog) that having square or rounded corners doesn’t matter in the slightest as you don’t see them.
PS: see full sized versions of the new AdSense ad unit styling options here.