As we previously described, the new system serves up ads based on keywords in Twitter search queries.
Promoted Tweets will appear at the top of the search results page, with small text indicating they were sponsored. The Times piece notes that companies could use this to combat negative tweets (they can place a positive tweet at the top of the page)
A Promoted Tweet isn’t guaranteed to stay afloat for a long time — if the tweet isn’t tracking well in terms of replies, clicks, and a number of other metrics Twitter is calling “resonance”, it will be pulled, and the advertiser won’t pay for it.
One ad will be shown at a time

Initial ad partners include Best Buy, Virgin America, Starbucks, and Bravo
Advertisers will be paying on a CPM basis initially, with plans to adjust the model once Twitter can better gauge how people are engaging with Promoted Tweets
So the long awaited news about the Twitter Advertising platform was covered yesterday in a number of online publications with the announcement from twitter expected today. (The above excerpt was from TechCrunch.com)
The programme is apparently going to be called 'Promoted Tweets', will mean that adverts will show up Twitter users search for keywords that the advertisers have bought to link to their ads and that in a later development, Twitter plans to show promoted posts in the stream of Twitter posts, based on how relevant they might be to a particular user.
According to comScore, Twitter.com had 22.3 million unique visitors in March 2010, up from 524,000 a year ago, excluding the millions of people who use third party applications to use Twitter.
The adverts will mean that as a business you will be able to insert messages about your products or services into the stream of real-time conversation.
In the New York Times there is a quote from Starbuck VP of brand, content and online Chris Bruzzo who comments “When people are searching on Starbucks, what we really want to show them is that something is happening at Starbucks right now, and Promoted Tweets will give us a chance to do that.”
Initially it is anticipated that you will will pay per thousand people who see promoted posts.
The interesting question for me is will we ignore the ads as some of us do on Facebook and in Google search results?
I wonder what the first ad insert in Twitter will be that I see today?
For additional coverage check out the following articles:
In AdAge
In the New York Times
UPDATE
Here is the official Twitter Announcement of 13 April