Replace 'em with some nice cross drilled plated rotors - they don't get any surface rust, and they perform better.
This is a proven fallacy, at least for an S2000. The OEM brake components on an S2000 do such a great job that few, if any, aftermarket rotors will perform any better. In fact, cross drilled rotors are ill advised for an S2000 that is tracked. They have a tendency to crack between the drilled holes in high heat situations. Cross drilled rotors look pretty but do little for better braking performance on this car. If you need better water shedding and off gassing, slotted rotors are recommended. The rust situation is pretty common on S2000s. As already said, they'll rust right after a car wash but will scrub off with the first brake application. Not a real big deal, but to side step this, you can get aftermarket rotors that are cadmium plated. They don't even have to be slotted.
The rust depicted in that picture is from being left unused for extended periods of time. One that is driven regularly won't ever look that bad.
Note: After a car wash, leave it in gear, but don't apply the ebrake. This will minimize the loud CRACK you'll get if you park it for a while and then take it out for a drive. The noise is the pads breaking free of the rotors due to the rust build up.