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:

http://pagead2.googlesyndication.com/pagead/abglogo/abg-en-100c-000000.png

Here's what it looks like with an ad having a visible border: