I posted that web visitors are not dim, and somehow my words convinced you I am thinking the opposite? Hmm... how did that happen? ;) Sorry, no disrespect, but that's kind of funny. :)
I believe you may be missing my point. People on the web tend to scan not read a page. If you sit them down and ask them to hunt for ads, that's not scanning, and it's completely different from how those same people read a web page when on their own.
YES, you can sit someone down and tell them to spot the ads, but that is not how they ordinarily approach a web page. It's like the difference between the way you normally drive a car on the freeway, and the way you drive when there's a highway patrol car behind you.
This is why web publishing has different page layout conventions than print publishing. Good web pages are generally delivered in small chunks with lots of BOLD TITLES to indicate what a paragraph is about-in order to facilitate the scanning process. There are hot spots on pages where a web visitor will quickly scan the page and leave if they don't quickly find their info. If you are unfamiliar with this phenomenom and not designing for it, then you are missing out on one of the most basic aspects of web life.
GP's highlighting of the advertising component of his ads facilitates the scanning process. When scanning you overlook little things like "Ads by Google," especially when the ad contains the exact information you're looking for.
So if you're searching for Wire Train MP3s, click on the SERP listing to the web page, and are immediately confronted with the same words:
It's going to result in a click.
But it may not be what you were looking for, and you back out of the advertiser's web page fast. By highlighting the ads from above with the words, Sponsored Listings, you are making it less likely that someone who is scanning a web page will unknowingly click through on an ad.