Is Twitter Making Money?
The short answer: no. I had thought they may had negotiated with the carriers to do revenue sharing. Pat Phelan thought they were using the various free email-to-SMS gateways in the US. The (official) Twitter Blog says they pay for SMSes just like everyone else, though they have at least negotiated a good rate with the carriers.
Twitter isn’t making money, yet. However, they do have a relatively low burn rate. I really have no idea how they are going to monetize what they are doing unless they offer some different services.