Paid Links, Google PageRank and Google Penalty

From, http://support.google.com/webmasters/bin/answer.py?hl=en&answer=66736

Buying or selling links that pass PageRank is in violation of Google's Webmaster Guidelines and can negatively impact a site's ranking in search results.

@mattcutts also posted on “Why did our PageRank go down?”

Here's a video on "How can I identify causes of a PageRank drop?"