It may depend on the car model, but you could always as the service dept. After a quick Google I found:
"[font=arial, helvetica]Answer: It is possible to machine the rotors too thin. Stamped on the rotor is a minimum thickness they should be machined to. If the rotors are machine below the minimum thickness, the brakes will not dissipate the heat and warping of the rotors can occur. When the rotors become to thin, they need to be replaced. If your rotors are thinner than the minimum thickness, they were machined that way."[/font]