Does this constitute a long-term solution to the problem for active communities? That is, default to rel='nofollow' for an untrusted class of users, which presumably includes all the spammers?
I'd say rel=nofollow ALL user generated content.. but hey, that's just me.
Not at all. I was addressing the fact that when a 3rd party site asks for links to be removed (and are un-willing to pay for 'time' it takes to take links down) and then threaten to turn you into the google police for link spam... you can defend your position and show that you indeed are not participating in link shenanigans.
that only works if the spammers realize their links are no longer effective; I'm not certain that that will actually happen...
If you have a relatively small site.. curating content isn't too bad... limiting the tail of articles so that comments can only be added for X days helps with that. Only allowing logged in users to comment (and making login easy via oauth sites like facebook/twitter/g+ etc) I kind of like Disqus, but would rather have something more home-grown.