I saw a take on the rel="nofollow" idea a while back that suggested that comment spammers are unlikely to change their practices as a result... comment-spamming is essentially a zero-cost option, so there's no financial/effort incentive for them to stop, and although they won't gain the pagerank mojo that they would have in the past they will still get some number of clickthroughs from the links left in comment spam...
again, this is only relevant to sites that allow anonymous commenting and for which automated comment-leaving scripts exist, so it's not likely to directly affect many (if any) OACS sites, but it's an interesting angle that I haven't seen explored anywhere else...
