To whit: users don't look at banners. Nielsen reports people do look at
* Plain text
* Cleavage and other "private" body parts
You can trick people into viewing ads by making them look like site content or like browser messages:
Yuck. We agree with Nielsen that such tricks should be avoided by ethical advertisers.
Nielsen on ad networks:
Ultimately, the fact that online ads get viewed more when they match surrounding content is a strike against the tendency to build advertising networks. If advertising spots are simply auctioned off, then you can't design an optimized ad for each placement.
When you advertise through an advertising network, your ads will get fewer fixations than if you contract directly with the publisher for a specific placement and design your creative to fit that spot. As a result, you should bid less for network ads than for customized ads that you place yourself.
link to full article: Banner Blindness: Old and New Findings