Selling text ads on a website, if they pass PR, is a major reason for drops in ranking.
While this is true, it is not always the case, and is often determined by the age and authority of the website in question. I see many top ranking websites in competitive genres selling pagerank-passing links and they have not been hammered with ranking drops or with traffic drops over the last two years.
Buying text ads that pass PR from other domains is also not so productive - the PR is being wiped out on the linking site so the ranking boost is no longer happening.
Again, not entirely accurate...it all depends on the location of the link paid for in context of the qualities of the website in question.