I wish laptop webcams were designed like this: when I want to turn off my webcam I slide, with my finger, a piece of plastic that physically covers the webcam. This simultaneously turns off the LED, which is always on unless the camera is covered by the plastic.

As an extra feature, firmware could hook up to the software that uses the webcam to send a "hang up" signal when I turn off the webcam. That way I don't have to find some button in whatever software the person I'm having a call with wants to use — I just "turn off" my webcam.