Twitter is going the way of Google AdSense and elevating the status of certain tweets sent from advertisers when users do a search. Twitter users will begin to see "promoted tweets" today, and the site will determine the success of the campaign later based on how the advertised tweets are used. Promoted tweets will function like any other tweets -- they can be retweeted and users can reply.
Twitter Co-found Biz Stone highlighted what he felt was an organic quality of the new service on Twitter's blog:
We strongly believe that Promoted Tweets should be useful to you. We'll attempt to measure whether the Tweets resonate with users and stop showing Promoted Tweets that don't resonate. Promoted Tweets will be clearly labeled as "promoted" when an advertiser is paying, but in every other respect they will first exist as regular Tweets and will be organically sent to the timelines of those who follow a brand.