According to Mashable:
Twitter has been aggressively courting advertisers in the last few weeks, assuring them that Promoted Tweets are no experiment. They won’t just appear in Twitter search or HootSuite, as has been the case for the past year. Nor will they be banished to the sidelines of Twitter.com, the way promoted trends are. They’re coming very soon, they’re here to stay, and the company is looking into ways to make sure we’ll see them — such as making the promoted tweets “sticky,” so they stay at the top of the page no matter how far you scroll down.
What this means to corporate marketers:
With this change you can “advertise” by having promoted (paid for) tweets appear in the member timeline. It is unclear on the impact will be. Clearly, the current position of promoted tweets on the sidebar has not been an effective advertising solution for Twitter. From our own data, we know that messages that appear in the timeline are highly effective when they are delivered by trusted sources such as when Gaggle Members share messages to their Twitter followers. However, the effect of adding promoted Tweets from unknown sources that the Twitter member does not follow is unknown.