I think people are taking the word ad too literally.
First thing that popped into my mind when I read this was certain words becoming hyperlinks.
http://en.wikipedia.org/wiki/IntelliTXT
The biggest problem with that is that it would be hard to tell the difference between a legit footnote and an ad.
Still, I wouldn't be happy to see it put into action. People can already wiki words and I don't want anything to take me out of the reading experience.