Twitter users are some of the most engaged - compulsive, even - users out there. Is charging money to have an account really out of the question?
As I mentioned in another comment, however, I think there's more money and a better sell to developers. Open up the API & charge for high-volume access to it.
I really don't understand what they're doing. Cash, users, attention, etc. are all there but they're not doing anything with any of it.