It is interesting all these sources tell you it is nothing more than a build up or deposit of pad material, but if you have your rotors turned to true them the lathe operator will show you the metal being removed to bring the run out down to acceptable levels. It is definately not pad material being removed, because if it was your rotor thickness would not be sustantially reduced they way it is to get a true rotor. Since I first started seriously tracking cars in 1989 using stock rotors and race pads I have found that every car since then (Mazda RX2 race car, Acura Integra R, C5 Corvette, Mitsubishi Evo 8, C6 Z06, and now the M3) all will produce a warped rotor after 4 to 6 one day track events, assuming you are pushing hard enough to get a lot of heat into the braking system. In my opinion rotors are consumable items when tracking your M3 just like the pads, brake fluid, and tires are.
2011 M3 Sedan
Dinan tune, intake, pulley
265-35-19 Hoosier R6's
for the track with Carbotech pads