This is pretty nifty. An ad network called World Web Network
Try the demo and see for yourself: it detects when you are below your screen's fold and can't see the banners.
I don't know if this has any real application in ad sales -- it matters little how "visible" the ad is if the average fixation (dwell) time for a banner is about one second --but I see it being useful in page content analysis. Too bad the tech is proprietary.
Correction: The technology was created by Alenty, not WWN who licenses it from the creators.