One complaint about rel="nofollow" is that you just might want to reward people who post related and constructive links in the comments section of your blog.
My answer to that is to implement reverse comment moderation. As far as I'm concerned, I'd like comments to appear immediately on my site. However, I don't want them getting Google juice. Call me vindictive if you will. I know it won't stop comment spam. But I do think it's a fine complement to other methods that do attempt to stop Spam
Instead, I'd be happy to go in and remove rel="nofollow" for all links within a post. Ideally there'd be a really nice interface for it. A simple checklist of the day or week's comments and I could check them off and post it.
Then again, I don't really care that much. If you really post a comment with something worthwhile, I'll just mention it in my next blog entry sprinkling a tiny bit of Google juice on your link. (And I do mean tiny. I'm part of that long tail Chris Anderson talks about).