One reason rotors can warp is a differential rate of cooling. One way this can happen is to use the brakes down a big hill - riding them all the way down, then coming to a stop, say a stop sign or a traffic light. So you don't move you keep your foot on the brakes. And there's the problem. While you're sitting there with the brakes on the pads come in contact with the rotor and that part of the rotor cools at a different rate than the rest of the rotor.

Better to downshift - even in an automatic - when going down a big hill, than to ride the brakes. Or if you do use the brakes try not to come to a complete stop so some air goes over the rotors and they have a chance to cool evenly. You don't have to move much or quickly - just so the pads don't set in one spot on that hot rotor.

We all understand brakes are cheaper than transmissions and for this reason downshifting with an automatic is generally discouraged. But, for going down huge or long hills it makes sense. There's a difference between doing it all the time and just doing it on some hills. And it's the shifting that casues wear on the tranny, not it actually being used as a brake.

Richard Sexton, Aug 06, based on obsevations by John Berryman