Fading Out Nofollows?
Seems like a good idea but wouldn’t just using Akismet, moderating all comments, and then not using no-follow at all also be a good practice.
Comments add value to a blog and although 90% of the comments you will get will be spam, most of the tools out there created to take care of the issue, will take care of most of this.
Why not give back to people who comment by linking to their url without the sneaky, underhanded no-follow tag that you have to look at source code even to find.