Most mobile phone cameras have an artificial shutter sound, designed to alert you to candid photography. It's not just a user feedback mechanism, or else there would be a way to turn it off. It is actually meant to protect other people from your camera. Here are some situations where it does not achieve its goal: loud places (concerts or even just a busy street), long distance, crowds (can't see), too quick (snap and run), deaf, distracted, listening to an iPod. And that's if you don't manage to turn the sound off through some other means. So the only real thing it has accomplished is annoying users.
Anyone who wants to take photos without you knowing will do so whether their phone makes noise or not. But all of that is not even the most serious problem, which is that the photo, by the time the noise is made, has already been taken. It's too late already. And these days those photos can be on the internet as fast as the button is pressed, so you can't even chase them down and prevent it going further with confidence. What you really want, in this type of alert system, is flashing lights and very loud sirens that go off for a full minute before the shutter. That way you can be alerted to the presence of the camera and take preventative action before the photo is taken.
Mokalus of Borg
PS - Which is, of course, ridiculous.
PPS - And would be impossible to enforce.