That said, I mentioned in another comment in another thread that I have to spend some of my time dealing with spam and all of its people effects on a pretty regular basis. I've also screwed around on the 'net since Mosaic, and dialed in to local BBSes before that (I started young :-).
In my opinion, any form of two-way communication which allows anonymous entry will be vulnerable to spam. I certainly can't think of a workaround for that, and nobody else has managed to yet, either. I agree that Twitter spam has a good chance of being eradicated once they tighten things up a little. However, in part they have that chance because Twitter isn't fully bidirectional -- there's no way that I know of for a Twitter user to send messages to someone who hasn't signed up to follow the user.
I still think the model is good. The follower spam problem described in those links (this too: http://www.stoptwitterspam.com/blog/about/) is indeed spam but could be could be fixed without it breaking the essence of the twitter approach.
I'm convinced that spam is a result of weaknesses in the model of the email system and not inevitable as you suggest. If we had our time again we would be able to standardise on a better model and for this reason was a bit surprised that you just accepted spam as part of life.
The blog link at the top describes some of my thoughts. Models that involve variable payments work as well because they directly price on how much you want to be paid by people who waste your time. In such a world I'd love it for people to spam me.