Actually, no it doesn't. All servos become undriven when they loose power - how else could it be? That fact that some fall by the weight of the surface on the ground depends on the surface weight, and also the type of motor and gear ratio in the servo - i.e., the amount of friction in the system. The only servo that will actually hold a position without power are those driven by a jackscrew, and I don't know of any of that type that are used in RC aircraft (and even in those, it really just a matter of a lot of friction in the gear train that is holding the position).

Bob