It seems problematic to charge for the basic functionality of something so simple; there are plenty of Twitter competitors that offer what Twitter does and more, still for free. (coughpownceahem)
They could charge for expanded functionality (again, removing API and SMS limitations, and possibly additional features), but I don't think they can take away what they've already given for free for so long, and I have to wonder how many people need more than what's already free so much as to pay for it.
I agree they should be very careful, otherwise they could be looking at a mass exodus. However, I think that with how popular they are, they could get away with a reasonable fee.
They could charge for expanded functionality (again, removing API and SMS limitations, and possibly additional features), but I don't think they can take away what they've already given for free for so long, and I have to wonder how many people need more than what's already free so much as to pay for it.
Just thinking out loud, anyway.