Breaking news! No, it's hardly worth mentioning, really, but every now and then I notice an extra horizontal line in Adsense ads. So far, I have only seen this on ads that have the same color set for the background and the border, so its purpose might be to clearly seperate the "Ads by Google" text from the ads when there is no visible border. I've seen it on more than one website, but it doesn't always show. They're probably just testing it for now.