Telling the valets was one of my first thoughts about the pushbutton. It's also an unusual feature that fascinates people, like the one-armed wiper.
This would also be very confusing to potential car thieves. Assuming one got in and realized the ignition key would not start the car, he would open the hood to look for the kill switch -- but would not find one. If he hotwires the starter he would then realize the car still won't start (because the fuel pumps are disabled too).
I am planning to install 3 or 4 switches that complement those already on the center console, so they will look original and not draw attention. My chief concern right now is a lock-out which prevents the starter button from being pushed when the engine is running.
It's an interesting project.