For an extended power outage trapped in a small area (subway or elevator) a chemlite could be more useful then a flashlight. Twelve hours of diffuse light could be more useful then keeping a flashlight on and running down batteries. More of a comfort item then anything, but for people who get claustraphobic (and I suspect that until a persons been trapped in a dark elevator they don't know how claustraphobic they can get) that's important.

Otherwise I agree with other people about carrying a water container, even if you have to carry it empty and flat. Water might be availible, but you want to be able to take some with you. I think StuToffee's idea of a small Platypus bag is great.