This is clearly not something that is going to happen in this lifetime from the looks of things. When given a recent survey, most people apparently voted to say that they would not be willing to subscribe to Twitter with any kind of payment. No dice, as it were.
I am skeptical of this, as I believe if properly packaged, most online marketers would gladly pay for the service. Obviously the key here is having some extra functionality to sweeten the pot a little. But at the end of the day, I do believe many business users would pay a fee if it provided the user with extra business related functions.
As for the casual masses, no, I am not surprised they would not being willing to pay. After all, Twitter is being kept afloat by people sending @ mentioned trying to constantly get people to try out their latest offerings. And this can be annoying. I mean, no casual user is going to pay to be annoyed all day. It wouldn’t make a lot of sense at all.