This is from a 2007 study on banner blindness by usability expert Jakob Nielsen:
On hundreds of pages [studied], users didn’t fixate on ads. The following heatmaps show three examples that cover a range of user engagement with the content: quick scanning, partial reading, and thorough reading. Scanning is more common than reading, but users will sometimes dig into an article if they really care about it.
(The areas where users looked the most are colored red; the yellow areas indicate fewer views, followed by the least-viewed blue areas. Gray areas didn’t attract any fixations. Green boxes were drawn on top of the images after the study to highlight the advertisements.) Read the rest at Alertbox.