The question of how Twitter will ever make money is on many minds.
For starters, Twitter cannot charge its users. Because all of Twitter is user-generated content, it doesn't make sense to make them pay. Paraphrasing the words of a co-worker: Twitter is doing nothing that can't be achieved using RSS and a 140-chars textarea :-)
The problem is compounded with the fact that Twitter has an API that can help bypass the site altogether, thereby ruling out the traditional - and failed IMO - online advertising revenue model.
What's left? A Twitter Pro with higher level of service?