OK...this is a fun discussion!! First, there are three things that happen to discs that people all call 'warped'- one is runout- truely warpage- not true with the flange surface, one is thickness variation- different thicknesses at different parts of the rotor, and parrellism(sp?)- the two faces of the rotor not running true to each other.
Third one is easiest- occurs when the ribs corrode and either A- swell and push the faces away from each other or B-collapse and allow the faces to come closer together. Can also be caused by heat.
Second one is what tom was talking about- actually, its not pad deposits that usually cause this- it's the pad ripping material from the disc. Usually occurs when the car sits for a while- with the metallic materials in pads now, it is a given that they will corrode to the rotor with very little time/moisture involved. You jump in the car, put in drive and RIPPP..... away goes some rotor surface(and pad surface)- this can be driven out of the car, at least on an MB, by braking hard, or using the sanding pads, if it's not too bad.
The first one is the real question- does it happen...yes- seen it, as too why- the best I can figure is this- (assuming things like rust on the hub have been cleaned,etc) when the lug is tightened, the cone of the bolt actually deforms the wheel cone to help hold it- you can't see it with alloys, but watch a steel wheel when tightening, if using an impact, when the cone distorts a little- the wheel is tight (and actually close to torque). So, it stands to reason that if the wheel is distorting, the rotor could too, actually I guess the hub could too, but you would have to be a freak to do that! As to the science of it- the pulling of one side tight while allowing the other side to lift pushs the lifting side against the hub with the center hole- hub holds still (obviously), and the disc's mating surface gets pushed outward at the edge of the hole-driving the rotor face away from the outside of the car- remember, we are talking about .0001ths of an inch here. Then that bolt gets tightened..and TAADAA, we have runout. What mark was saying was right, without the other force (the sideways push of the hub against the center hole).
Having said all that- the biggest problem is sloppy techs (usually young), not being taught right, having the tool guy sell them a new 'Thundergun' (because they are the best, which they are, but dangerous in the wrong hands), and you have a recipe for disaster!! I could probably do wheels up with my gun and not have a problem(and have), but I don't usually, just because it's ingrained to torque them now, and it also sets a real bad example for the young techs.