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

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.