The 2008 MBP did use PWM backlighting. PWM is slightly more efficient than a constant current regulator and gives a more consistent backlight color temperature. IIRC Apple have stuck with PWM backlighting for these reasons, but have moved to an extremely high frequency (~120kHz) which should be imperceptible even to the most sensitive users. Many other top-tier laptop manufacturers also use PWM backlighting, but with a switching frequency of around 1kHz.