This is old old old news but I forgot to add it here – Jeremy gave his diagnosis of the implementation of the nofollow attribute to links in blogs.
Nofollow has done … nothing. Comment spam? Thicker than ever. Its had absolutely no effect on the volume of spam. Thats probably because comment spammers dont give a crap, because the marginal cost of spamming is so low.
and the effect it might have on commenting
if your blog uses the nofollow tag, youve just eliminated that tiny little bit of reciprocity. Thanks, but no thanks. Id rather just comment on my own blog. And maybe, if youre lucky, Ill link back to you.