I was just looking through my server logs to try to see what kind of searches were bringing people on the site. Instead, I saw a whole bunch of inbound links from a particular high-level domain that apparently teaches people how to play poker. Doesn't make sense; I like poker, but there's no way this blog should be a poker-player's destination. It turns out that I am being "referral spammed." Some blogs automatically publish lists of referrers (that is, inbound links that have been followed). If I were to be do so, I would be publishing links to this particular poker playing site and thus driving up its PageRank.
I find it interesting that I haven't yet been discovered by the comment spammers, and my referral spam appears to be limited to just this one high-level domain. I imagine that once you get on a "vulnerable" list, it's all over... If so, this would argue for the efficacy of "nofollow." Presumably, if you managed to implement "nofollow" before getting on a list, you might not be marked as vulnerable.