If you go by the story from SEOMoz, there’s too much fraud in the advertising arena and the only format that makes sense for advertisers is flat-rate models such as Text Link Ads (aff).
The post writer, Michael Martinez, cites a staggering fact that 14% of clicks in the click-through variety of advertising is fraudulent. I don’t know where he got his numbers, but I have no reason to doubt that they are accurate.
I’ve thought for some time that contextual advertising is dead, and I see the argument about impression based advertising. But could his prediction be true? I sure hope not.