I think it was on Matt Cutts blog that he mentioned any links to advertising should have a nofollow, to protect your site from being seen to "sell links", which would then result in a penalty in the form of not being able to pass page rank, and having your page rank reduced.

What actually happened was on the previous update google experimented with a PR hit on sites that were selling links. It was specifically targeted to sites who were doing so with intentions of altering the SERPs. Thus paid links that had specific keyword specific anchor text. Most of the sites on the Internet were affected. Sites with organic links, I.e links that were plain advertising without intentions to manipulate the SERPs were not affected! On this last update the vast majority of the sites which had a reduction regained their lost PR. Instead this time round Google have implemented another strategy which looks highly effective.