12 hours after I first heard about the nofollow idea, I have to say I'm 100% happy with the idea. Last night I made a dumb jokey comment that they should have used nofollow in the links on the blog, not knowing it was aimed at just comment areas. I also had a first reaction of "this won't solve comment spam overnight" which it won't, but that's fine because it will make Google better by giving it better results today and maybe curb spamming eventually.
I have a love-hate relationship with pagerank in general and more specifically with the inflated pagerank given to blogs. But when I got a crazy high rank for PVRblog (I think it's gone down since then), I felt an obligation to keep artificial URLs away from the site.
My premise was that if Google thinks I'm an authoritative source of information (even if I'm not sure I deserve it) and that my posts are highly topical and help people searching for information, then it was my duty to keep the information neat and tidy, and keep people from cheaply trying to take advantage of my position. A nofollow feature would be perfect for PVRblog since I didn't want my good article on how to setup a TiVo without a phone line to get mucked up with people trying to sell satellite systems or even worse, viagra pill offers.
And someone on MetaFilter explained it well with the old adage you hear from anyone that ever built an application used by many: never trust user input. That's what nofollow does, is allow site owners to maintain their sites with good information they deem worth posting about, while at the same time keeping user submitted data apart from that, to be viewed by search engines as "here's the stuff I wrote and some other people added stuff here below that you can feel free to index as text, but ignore any links they add since I can't be an authority on whether they provide additional information or not."
Sounds about right to me.