He'd be a great pickup, but the Braves don't like to deal within their division (they'd hate to see the players they developed beating them down the line all too often in an unbalanced schedule).
But they want an elite pitcher, and there's only a few out there. Greinke is more likely. The Hamels signing just made that less likely though, as the Braves would probably want to trade-and-extend, but Greinke's value just went up after Hamels' ridonkulous contract extension.
ETA: He'd (Johnson) be under team control in 2013, which makes the Braves a little more willing to trade for him, but makes teh Marlins less likely, as they will be reloading trying to win the division next year, and won't want to face him in-division.