According to the Twitter Terms of Service, just updated this week:
"The Services may include advertisements, which may be targeted to the Content or information on the Services, queries made through the Services, or other information. The types and extent of advertising by Twitter on the Services are subject to change. In consideration for Twitter granting you access to and use of the Services, you agree that Twitter and its third party providers and partners may place such advertising on the Services or in connection with the display of Content or information from the Services whether submitted by you or others."
What? Allowing ads on the service? Twitter, say it isn't so. This won't really affect those that use third-party clients, such as TweetDeck, Seesmic and DestroyTwitter (unless the powers that be decide to inject them into your tweet stream, which will surely kill the service). But anyone visiting the webpage to post (like I do from time to time) will surely be bombarded with advertising.
A la Facebook?
I hope not.