Looks like Google’s starting to look at how blogs and spam interfere with search engines and indexing.
On the plus side, this will help cut down on spammers who generate a high rank by spamming blogs. A recent search on a particular quote from Plato turned up pages and pages and pages of personal blogs where that quote had been used as part of some spammer’s random text corpus.
On the other hand, it’ll reduce the rank of normal commenters and their links back to their own homepages and blogs, too. Whether that’s a good thing or a bad thing is probably debatable; some people argue that blogs are overrepresented in search engine results already, but someone who regularly comments on-topic in various places might deserve a bit more exposure…
I’m a little worried about how nofollow will change search engine rankings. But not as much as when I first heard about it. The good search engines had good information before the blog boom. I don’t see why they won’t after nofollow.