In my anecdotal experience (about 60 or so flights a year) AirTran is prone to close the door as early as they can -- 10 minutes before the flight is scheduled, I believe it is -- as often as they can.

Originally I thought they did it if they saw from the tickets collected that everyone was already on the flight. But I guess they might just do it whenever they can. It is, I suppose, possibly that they'd shut the door earlier than 10 minutes out, if tickets showed everyone was on board (which would only then effect unscheduled people trying to get onto the flight).