Watching Twitter and app.net over the next couple of years should be fascinating. Twitter can't start charging, but app.net is starting there. If they get 1% of Twitter's population, they are a huge success. And Twitter just gifted them a lot of potential developers.
They could easily start charging for additional services as the comment above suggessted - if they make it a 'Pro' plan and leave normal accounts alone, there's no incentive at all for people to leave, and yet they make money from their most active/interested users. There are plenty of extras they could offer on top of the basic service for a charge.
That goes double for business accounts - I'm sure most businesses would pay some small subscription for verification, syndication, the chance to advertise (in clearly separated areas) and other features now it has critical mass.
That's a much better plan than polluting all of the users' streams with advertising they don't want a la FaceBook, which will lead to a mass exodus. Adding advertising to the users' streams is absolutely the worst thing Twitter can do, and yet it seems they're actively working towards that with their recent changes.
Although I wouldn't be surprised if the knuckleheads at Twitter botched it.
Twitter and Facebook could learn a lot from the Free-to-Play video game market. Service is free, being "special" costs money.