This is something I've wondered about for a long time. On really large airplanes, like a 747-400 or A380 for example, I would assume that the weight of the passengers on board is a significant to the flight characteristics of the plane. And it seems like it would be pretty easy to put all the passengers in an odd place (like all at the front or back) and as a result significantly change the flight characteristics of an aircraft.
So, assuming I'm right (tell me if I'm not), how do commercial aircraft counter this problem? Do they do their best to distribute passengers throughout the craft with seat assignments? Or do they have ballast they can move around (like fuel ballast)? Or is the weight shift not significant enough to warrant anything along these lines?