Emergency lighting (Score 2, Interesting)

How about hooking up some low-power emergency lighting around the house - even LEDs would be useful to let you find your way around. You could also tap into the mains ring, so if power drops a small set of lights could come on. You might even be able to neatly recess some small bulbs into your skirting, or lower down in the wall. I'm sure it would break the rules on any service plan with your landline provider, and may even be illegal, but if done well it would be very cool and also pretty useful.