footprints" behind in the code

The good news for search engines is that most text link ads and the sites that sell them tend to leave noticeable "footprints" behind in the code. It would be no trouble at all for a search engine to do a little digging into what the latest footprints are, search out all pages that have them, and simply not allow them to pass any link popularity. This is not a penalty, mind you. It would just be a way for the search engines to count only votes and not ads. Your ads would still be worthwhile for the exposure and direct traffic they bring, but not for providing you with link popularity. So although your site wouldn't technically be penalized, its rankings could drop if it was dependent upon the link popularity of paid links.