Uhh, 'no-follow' benefits Google et al., not site owners nor this Google user, and it's doubtful that comment spam intends to boost pageRank in the first place. I hate it as much as anybody, don't get me wrong. But if my query on "wordpress nuances" starts to feature a link to "BonerPill.com" on results page one, I'm still not following. Probably. And comment spam costs nothing to do. I'd like to meet the comment spammer who ever says, "Damn, my pageRank isn't getting anywhere. Guess I'll call the robot home."
Anonymous commenting invites comment spam. Comment moderation or commenter authentication beats it. Some things can't be automated.
Seems to me, then, that the no-follow attribute value should be an opt-in/plugin proposition, for site owners who'd rather cripple the Web than moderate. Search engines and site owners don't have common cause here, not really.
Unless Google's response will be to devalue links on blog-style sites (except Blogger.com sites, natch). But bowing to such threat invites a new tyranny.