Richard at SEO Gadget showed how Google seemed to have penalized specific pages of his site from ranking in the Google index. The penalty seemed to be fair, in that there were nasty comments that slipped through his comment spam filter.
The drop in traffic can be seen by the keyword phrases that page ranked well for. He noticed a ~70% drop in traffic for that phrase, which in his case resulted in a 15% drop in his Google traffic and a 5% drop in overall traffic.
What I find extra fun is that a Google Search Quality Analyst, @filiber, tweeted:
Google Page level penalty for comment spam – rankings and traffic drop http://bit.ly/JNAly (via @AndyBeard) <- interesting read!
Of course that is not admission to this as a fact, but it wouldn't be too hard to believe that bad comments caused such a decline.
Now, I don't think this would be considered a keyword-specific penalty, which most SEOs believe in, but rather a specific page being penalized.
Forum discussion at Sphinn.