The relevant part of the Terms reads as follows:
"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."
According to a report on Bloomberg.com, Twitter plans to begin introducing ads on a limited basis in Q-4 this year. No pricing models or revenue projections have yet been released.
Meanwhile, Compete.com reports that Twitter's web traffic grew to 23.58 million in August, making it now the 38th largest website in the U.S. Bear in mind that this does not include all of the users who access Twitter by mobile devices, which some research suggests is a larger number than those who Tweet from computers.
For a more in-depth look at Twitter's advertising moves, please read my Bnet colleague Cathy Taylor's post.