I just did every search (Taj Mahal, Marie Curie, Matt Groening) they did on my unaltered Firefox/Windows 7 install while logged in, am certainly not a Google employee, and got no ads. Also didn't get the Knowledge Base yet, if that is a factor. Famous locations/people searches don't seem to be stuffed with ads typically. I'd imagine more research style searches having ads would be a bad experience for both the user and advertiser compared to stuff like "cheap flights" which does have an amazing clutter of ads. And of course, picking research style searches presents the idea better. Lack of ads is a nice bonus.