degen It depends, some laptop panels have the PWM generated in the panel itself, given a brightness value, some have it modulated by the motherboard, etc.
In the near future I may be buying a cheap netbook with PWM to test my idea of an external device that "upscales" the PWM frequency from the low frequency to a higher one that is "undetectable". Based on my review of laptop panel datasheets, this idea seems plausible.