I just noticed Adsense ads which show Payperpost.com on DigitalPoint. Have they been there for a while? This just goes to show that Google does not have a consistent / uniform companywide policy on what is right and wrong when it comes to gaming the system. On the one hand, you have Matt Cutts speaking in public forums and in his blog that paid posts should have a nofollow on the links. And Adsense at the same time allows ads by payperpost, with ads which claim that what you get are backlinks. Payperpost, of course, doesn't allow you to use nofollow in any of the posts for which it pays. (Or am I wrong and they do?) If I am right, heres another instance of the right hand not knowing what the left hand is doing.
When Matt Cutts speaks, he is reffering to what the search-index likes or does not like. Also, usually when he says these things, or blogs, they are "his personal opinion" - not "Google law"... Just because he makes a comment hinting at what is or is not good for your SEO does not mean that the Adwords people are going to change their policies. I see no issue with Adwords allowing people to pay to advertise things that the search-index team does not like.
Payperpost isn't illegal so why should Adsense ban their adverts? Matt Cutts has always said paid links should have a nofollow but that doesn't mean they will penalize sites that want to use Adwords to advertise link sales.