Everyone said that nofollow wouldn't have any effect on comment spam, that it was a silly proposition. Although I can't say generally, obviously, I personally have seen a huge amount of benefit.

Before nofollow, I was getting ~5 comment spams a day . Since I implemented it (by upgrading Wordpress), I haven't gotten a single one.

It's possible it's just a random change. But hey, It seems to be working to me.