Matt Cutts [Google God] has given strong hints that participating in controversial programs like 
ReviewMe or PayPerPost, that pay bloggers for writing about advertiser’s product or services, may jeopardize the search engine rankings of your blog. This also applies to websites selling paid text links on their pages.
Speaking to Mike McDonald, Matt made the following comments on paid text links and paid blog posts: Paid Blogging Could Harm Your Google Search Rankings, Use Rel=Nofollow » Digital Inspiration © Amit Agarwal