We've noticed some new visual elements in AdSense ads - namely, Google has been adding enhanced graphic elements where the old "Ads by Gooooogle" text used to appear.
They are using transparent PNG files to overlay the border graphic.
Here's a sampling of the new graphics:
Image may be NSFW.
Clik here to view.
http://pagead2.googlesyndication.com/pagead/abglogo/abg-en-100c-000000.png
Here's what it looks like with an ad having a visible border:
