Last month, Matt Cutts gave strong hints that selling text links ads or writing product reviews for money could harm your blog rankings in the Google Search engine unless you embed the rel=nofollow tag:
You want my links for traffic.. totally fine..just don't make it so they affect search engines..so that's why we say use nofollow..or use a redirect which is through robots.txt
Jason now points to another comment of Matt where he has reiterated the whole thing in a much clear fashion:
Google wants to do a good job of detecting paid links. Paid links that affect search engines (whether paid text links or a paid review) can cause a site to lose trust in Google.
The outgoing links from your website / blog is a factor while Google calculates the relative importance of your site.

Our suggestion - Always play nice with Google since that will help you (and your site) in the long run. Infact, it is very tough to match the advertising revenue generated from search engine traffic with regular or return site visitors.

The future of "paid blogging" sites like ReviewMe or Payperpost is doesn't look very bright anymore.

Related Tutorial: How to Use Rel=NoFollow ?