Any gas appliance has the potential to either leak gas or burn inefficiently (resulting in carbon monoxide). Our stove has electronic ignition on it, but as a PP mentioned, it'll still leak gas if it's turned on and not allowed to ignite. Anybody who has any sort of fossil-fuel-burning appliance, whether it be for heating, cooking, or otherwise, should have a CO detector in every level of their home. As for a gas leak, I certainly hope most people would notice it, since gas does have a stink added to it (which I think is a low-tech but elegant solution to a potentially nasty problem). Natural gas is normally odorless.
We use the knob covers on our stove, and they seem to work well. There're also stoves with the knobs at the rear, which seems like a good idea to me.