For those who are concerned and unaware, we do count attendance by sales. This is straight from Marty Bodnar, Director of Ticketing/Marketing, to whom I sent a (now) embarrassingly long email about saving the 100k attendance.
"Our announced attendance is based upon tickets sold. Since we are close to a sell out of this game, we are not in danger of dropping below 100,000. Thank you for your concern. Go Blue!"
So that's... a relief? I guess I wished we counted scanned tickets, though this has to be a lot easier to measure.