I'm afraid I can't say much beyond what I've already said, except that Google places a fairly high value on detecting fraudulent activity.

I'd be surprised if I discovered that no bad guys have ever tried to simulate the search behavior of unique users. But (a) assuming those bad guys are a problem, I strongly suspect that the folks worried about search result quality are already on to them; and (b) I suspect bad guys who try such techniques give up in favor of the low hanging fruit of more traditional bad-guy SEO techniques.