I think that in the long term, the new nofollow tag is going to be one of the driving forces in killing off that irritating new phenomenon known as comment spamming. There are a variety of other good ways to kill off comment spam, but this one is effective, because when it is implemented, it makes comments completely irrelevant to search results.
This may change the popularity of some blogs in unexpected ways, but overall, I think the nofollow tag will be a welcome addition in the arsenal of anti comment spam tools available to serious bloggers.