When I was at Gnomedex, I was surprised at the general dismissive nature present towards making money towards web / online advertising. Dave Winer elaborates a bit here: Making money with ads? Not much longer...

Perhaps there needs to be a more deeper analysis of what a person wants and what's going on for them in order to even begin to offer them a set of ads relevant to what they are looking for.

That kind of attention tracking - data mining - crosses a scary boundary. What's the benefit to the user? More crap to consider buying? How do you get the ads to more or less be on their side and relevant? If there was a relevancy quotient associated with most ads today, 0.0 to 1.0, how many would be hovering near zip, zero, zilch?