In a long anticipated move, Google is now taking action to web sites who provide paid links. Google doesn't penalize the relevancy of the site in search, but rather, hits the sites in their pocketbook by penalizing their page rank. If the site has low page rank, which is generally a requirement of paid link services like PayPerPost.com, then the site starts to lose out on advertising dollars.
Danny Sullivan wrote in detail about this new development in a post at Search Engine Land, entitled Official: Selling Paid Links Can Hurt Your PageRank Or Rankings On Google.
"...Overall, the move takes Google into a new era of attacking paid links, allowing it more precise weapons than it has had in the past. For example, both the Stanford Daily and New Scientist are among several prominent sites that sell links. Google has not really been able to penalize such sites as that would hurt core relevancy. People expect them to show up.
By using PageRank decreases (something Google first experimented with in the SearchKing case in 2002), Google can hurt the perceived value of buying links from a particular site without harming core relevancy."
Bottom line: if Pagerank is important to you, you need to carefully consider buying -- or selling -- paid links.