Good point. But I'd also be interested to know the experience level of users involved in Nielsen's study. My own anecdotal experience suggests that people less accustomed to the web are far slower at discerning what is an ad and what is content.

On a related topic from that link, I don't think I've ever come across a banner ad made to look like a dialog box that has been good enough to fool me. But again my experience seems to suggest that those with less computer experience are far more easily fooled.