It looks like the advertising model for the web has some of the same baked in delusions as the advertising model for TV has: everyone assumes that the numbers mean something so that they can go forward:
Web sites that rely on advertising -- including some the most poplar, such as Google, Yahoo, MySpace and YouTube -- get paid based on the amount of traffic to their site and the number click-throughs on their ads. Without accurate data, advertisers have no idea how much they should be paying.
The only reason the system isn't breaking down, and advertisers aren't pulling out, is because they have no choice but to play. They are taking informed guesses, based on the shoddy statistics available. And Google et al. are using every strategy they can find to deal with this problem.
The introduction of DVRs started to punch holes in the TV nodel - once it became clear that people were skipping ads, the shared delusion got harder to maintain. It's still there; money has not (yet) dropped in TV-land.
In reality, it's questionable whether people ever paid much attention (even more so once the remote came along). Last night I flipped between "Band of Brothers" on the History Channel and the Yankee game on ESPN, changing channels at ad breaks. The same thing happens on the web. Take Salon, which allows you free access if you "watch" an ad. I use Firefox, which has tabs. I hit the ad, then flip to some other page. I go back after I finish, hitting the "enter Salon" link. The advertiser sure isn't getting much from me, even though the ad was delivered.
I suspect that a lot of web ads are "viewed" this way. Which only makes the accuracy of data problem worse. I count as a viewer of the Salon ads (and sometimes, by mistake, as a click through - although I simply close the window/tab in that case). In the grand scheme of things, I don't really see a strategy that deals with this.