First, Twitter signed a partnership with Google and Bing. The search engines pay Twitter in order to index the tweets. This agreement enabled Twitter to be profitable. (see the article) But this is not the end.
Twitter offers now sponsored tweets. If you use Twitter's search engine, you might have noticed. The first sponsored tweet was bought by Starbuck.
Twitter has also announced they will provide official client. This means that all client that work today might be out-passed bu the future official client. Tweetdeck, Seesmic... all those companies that helped Twitter to grow are basically fooled.
It's a great move by Twitter. The company has learnt how to behave in a competitive market. Sure, it might be unfair for the old Twitter client. But they made a great mistake. They relied on a single company. Huge mistake. (cf a previous article)
Let's just hope that the user experience will not be damaged by Twitter's gold rush.