Source: "Banner Blindness, Old and New Findings", Jacob Nielsen, Aug 20, 2007
Jacob Nielsen confirms that people don't see online banners: "At all levels of user engagement, the finding is the same regarding banners (outlined with green boxes in the above illustration): almost no fixations within advertisements."
But that's Ok. ArsTechnica writes, "There is a long history of experiments that show that repeated exposure to a stimulus that's barely perceptible can enhance a person's feelings towards what's otherwise a neutral object." And now there is a new research that concludes that "repeated exposure to a product via banner ads generates a positive feeling towards that product." (Note: the ArsTechnica's article came out in May 2007, three months before Nielsen's).
Conclusion: buy cheap remnant space. Nobody sees the ads anyway, but the subliminal effect is the same.
Study: Banner Ads Affect Memory