I've wonder about this - I've seen cars with the traditional cruise control (ie vacuum servo, actuating the throttle, and so moving the pedal). I've also heard about later generations of the same car / engine with electronic throttle, where they still used the vacuum servo.
Dunno whether that's accurate, and if so, why.
I've also heard people discussing cruise control in modern cars with electronic throttles, that have found it disconcerting - as in those instances, the cruise control action wasn't reflected in the throttle pedal position, and they found it odd when they wanted to temporarily accelerate.