Your original question lead towards a great solution with the streamlight light box - a 20W wide area beam rechargable in its cradle, wich starts the light when the power goes out. I like the idea that after mounting it to a wall somehwere it would likely stay put - and be easily found if it grows legs and walks.
Another entry into the rechargable scheme that I'd consider is the surefire 8nx - with 2 extra batteries, a fast charger, and the secure lamp, it's a good value at under $100
I don't think they have any sort of auto-on when power fails capability.
Let us know which you decide on - good thoughts there!