In what rooms of the house does it make sense to change all light switches to motion sensors? I think the only place it wouldn't make sense is anywhere you want it to remain dark - the bedroom and maybe home theatre. Everywhere else, it makes more sense, to me, to have motion sensors so that the lights are only on when people are present, and, furthermore, come on automatically when people arrive. I've seen motion-sensor lights becoming more common in offices. I think they'll start being used in homes before too long.
Mokalus of Borg
PS - You probably still need kill-switches for when you're going to change the bulbs.
PPS - Or else it would be like a heist movie every time as you try to move slowly enough not to set it off.