It is really an ad, no plugin or other trick. I also noticed this two days ago, ...

gaoprea on Nov 24, 2011 | parent | context | favorite | on: Google showing Ads DIRECTLY in Google Chrome

It is really an ad, no plugin or other trick. I also noticed this two days ago, I have a screenshot with it on my blog (link is on my profile, not posting it here). After a while the ad seems to be disappearing for good.