With warped rotors I would've expected some movement in the steering under normal braking as well. Perhaps the ABS is operating alternately between the front wheels which may cause alternate pulling in the steering from one side to the other. I've noticed that ABS in some cars operates quite smoothly while in others it creates quite a severe vibration. The ripples in the road certainly won't help under hard braking as the tyres will be experiencing varying levels of traction. Perhaps you could test your car on a smooth piece of road and see if it still does the same thing and try to notice if the brake pedal is shuddering at the same time as the steering wheel.
"Respect my Infinita!" - I like it LOL
