"The most scientific such stab at judging the Stewart crowd appears to have been carried out by CBS News, which hired Digital Design at AirPhotosLive.com to do the work. They came up with a figure of 215,000. Digital Design uses cameras on tethered balloons to take high-resolution photos of a crowd. Then they enlarge the photos and begin counting. They try to figure out the density of sample crowd areas. Then they extrapolate from there based on how far the crowd spread. You can read about how one of their estimators goes about the work here"
Sounds like a fairly reasonable method to me.