Twitter is a PR / Word-Of-Mouth Advertising type of company, when it comes to how they could make money.
One thing that they must have been thinking about for a long time is this very simple concept: sell paid tweets on Twitter. This might be the holy grail for Twitter. How would this work? Each paid tweet would show up with a little logo, so that the user knows it's paid content. Paid tweets would be created by advertisers on their Twitter or Feed Burner account, and would then be integrated into Twitter lists belonging to Twitter affiliates participating in this program. Advertisers would choose where they want to show up - for instance SAS could decide to advertise on our AnalyticBridge Twitter list.
Feel free to contact me about this business model - I know I can sell Twitter (or other) feeds to my advertising clients, and I don't even need Twitter.com to do that. There is some great potential in this business model.