Google doesn't appear too worried about impression spam--defined as: ad impressions generated outside of normal search activity, in some cases deliberate in other cases not deliberate. I don't know if they say they're not worried because they don't care, or because they feel like they have a handle on it.
There's an article on clickz about it today. I assume most Google advertisers are concerned about the issue. I know I am. I don't want to have to pay a higher CPC just because some jerk jacked up the impressions for my keywords. I think Google's CTR-based ranking system is great in a perfect world--that is in a world with no cheaters. The problem is, there are a lot of dishonest, unethical marketers online who will do what they can to cheat the system.
I actually like the idea of having the ad's placement based partly on CTR. I just hope Google has some reliable checks in place to stop impression spammers. I hope they really are able to keep up with the spammers on this issue as well as click fraud and all the other issues that keep coming up. It would probably be better if we'd keep quiet about it so we don't give spammers any ideas.
Oops. Me and my big mouth.