According to the Roberson piece, the announcement (although not official) would be in the neighborhood of April 16th.
The league and prospective owner Arthur Blank have been dancing around the topic for a few years now. All the attendance figures that have been accumulated over the friendlies and exhibitions have been a good carrot to dangle under the nose of the "Soccer Don"- MLS Commissioner Don Garber.
Miami, along with David Beckham's cache and ownership, are in the queue and the league would like to get to an even 24 franchises in the next five years or so. The team will be looking to play in the new and improved Georgia Dome by 2017.
Adding Atlanta would give the league another top-nine media TV market and another team in the under-served southeast.
As an example as to how the market responds to international football (the Mexico-Nigeria friendly), here's what happens when 68,000 show up...
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More when we know more...