Google, as it appears has aggressively begun to crack down on Ads that violate Google's quality guideline of "paid links should be disclosed through a "rel=nofollow."
According to Matt Cutts "By the way, at SES San Jose I got feedback about "buy PageRank"-type ads in AdWords. I recently chatted with AdWords policy folks, and the AdWords folks have disabled ads for many queries such as "pagerank 8" that would have shown ads before. I expect that we'll probably disable ads for more "buy PageRank"-type queries as well.
Thanks to the people who provided that feedback at SES San Jose, and I'm looking forward to chatting with people more at the PubCon search conference next week as well."