I suspect I would pay at first. But I imagine the introduction of an overt commercial face to Twitter will firstly, significantly alter the quality of the community and secondly, provide a huge boost of momentum to the upcoming technologies like Identi.ca and Diaspora. It would only be a matter of time before the centre of gravity for consuming ambient intimacy would move away from Twitter and where upon I would also relocate.
Personally I feel this theme relates to the broader and more fundamental issue of how identity is represented on the Web (I wrote more about it here http://www.tombh.co.uk/what-if-facebook-was-as-open-as-linux). Ultimately I predict that we will eventually settle on a truly distributed and federated mechanism for both asserting and expressing identity, much in the same way we recognise other standards such as HTTP, HTML.