Furthermore, the search engines could nip this kind of abuse in the bud, by ignoring the "nofollow" attribute of the links on a site if it determines that site

Furthermore, the search engines could nip this kind of abuse in the bud, by ignoring the "nofollow" attribute of the links on a site if it determines that site is abusing it (e.g. if all or some high percentage of the links on the site have "nofollow").