Technology Review: "A new study suggests that marketers shouldn't fixate on the number of people who click on ads. According to the research, just seeing an ad on a Web page can impact memory. Subjects who paid attention to a banner advertisement were more likely than those who didn't to recall whole words and facts from the ad--facts stored in explicit memory."
The study is published by an assistant professor at the University of Kentucky Chan Yun Yoo in Journalism and Mass Communication Quarterly, Spring 2007. I can't find the full text, but see his doctoral dissertation "Preattentive Processing of Web Advertising."