David Chartier has a few ideas on how Twitter will make money as a company and he also briefly chats to Evan Williams from Obvious co. / Twitter Inc. about it. The quick overview of the ideas are:

1. Twitter Pro - Charge for some upgraded services
2. Merchandise - Sell stuff
3. Twitter for Business
4. Ad supported Forums
5. Advertising
6. Sponsorships

Also this comes to mind. Twitter must be giving the phone companies huge revenue in SMS charges. Ev has said in an interview (can't remember where) that they are actually paying these bills at a discounted rate. These are only the updates they send out. How many more people are sending updates TO twitter and generating revenue for the phone companies. Maybe there is a way to share in some of these profits. Difficult but possible if they can prove the volume of these messages.

The last way off the top of my head is data mining. They have all that data, they must be able to use that somehow. Someone must want it and find it useful if it is sliced and diced and presented nicely.