I agree with the idea, but not with feeling like an idiot if someone else thought of it first.

I have an electric stove which requires a timer be set to permit the hotplates to be active. The system has saved me several times from a hotplate being active when there is nothing to heat. It could probably also work on a gas stove with a timed valve system.

This will become more important as energy costs increase. However, stoves are rather inefficient in any event, and will be eventually replaced by microwaves, slow cookers, etc.

The only problem I see with a weight activated switch is it will shut off if you lift the pan to flip whatever you're cooking. Maybe a time delay of 20 seconds or so could solve that.

Inductive cooktops can sense if an appropriately metallic cookware is present.

Definitely something novel. I can't think of a reason to keep an element active if there's no place for the heat to go. I also don't see the need for timers—the element would only be active when both the source (gas/electric/other) is active and when weight is applied. You might need a "weightless" setting for extra-light cookware or to sear things like tortillas, banana leaves, or peppers.