I like Twitter. I post a reasonable amount (twitter:parkylondon) and would be prepared to pay for my use of Twitter.
My suggestion (FWIW) is that if you post less than (say) 30 times per month it stays free but over that you pay (say) five bucks per month. If you are a mega-poster that would rise to twenty bucks a month for over - say - 250 posts per month. The model is from Libsyn - it works for them, and me, and I think it would work for Twitter.
If they get people dropping off then so be it. It's a great service. Let's make it pay.
This was originally posted as a comment to this posting at Read Write Web.