Google penalizes sites for comment spam links to bad neighborhoods, but lifts the penalty once comments get cleaned, according to Richard Baxter, who wrote a case study on his own experience to
share with the SEO community.
While he was away, traffic dropped on Baxter's site. So he drilled down into Google Analytics to find the reasons why. Drilling down to the keyword level, he found the culprit: comment spam missed by Akismet, which typically filters out the bad stuff.