What if you picked up the telephone and instead of getting a dial tone you got an ad?
What if you picked up a dictionary and instead of finding a definition you found an ad?
In both cases you'd be angry. Why? Because the telephone is a medium of communication and you don't want to be slowed down. The dictionary is a medium of information and you don't want to be sidetracked.
On the other hand, you turn on the tv every night and the first thing you see is an ad. It may be mildly annoying, but it doesn't make you angry. Why? Because tv is primarily a medium of entertainment, and you are accustomed to entertainment media carrying advertising.
The web is a little of each -- part communication medium, part information medium, and part entertainment medium. However, because the web started as a medium of communication and information, we learned early to resist being slowed down and sidetracked on the web. That's why display ads are so stunningly ineffective.
The ironic thing about the web -- new age marketing theory notwithstanding -- is that it won't be an effective advertising medium until it more resembles television. That is, until it is more of an entertainment medium and the advertising is embedded in the entertainment.
On The Other Hand...
Search is effective. But is it advertising or something else? Stay tuned.