Banners get ignored - Nielsen. Use them anyway - Ars Technica.

Users rarely look at display advertisements on websites.
Banner Blindness: Old and New Findings (Jakob Nielsen's Alertbox)
We knew that, but this drives it home.

So should advertisers stop using them? No, because even the most marginal impression, repeated enough times, achieves a positive impression:

"...even minimal exposures can create an actual positive evaluation (for example, the conclusion that the item is not a threat). That positive affect then influences future evaluations.".

More info here.