did you only notice this since you put one of those energy saving little fluorescent bulbs in? if so, then the other reply is correct, the neutral is switched and not the live. ordinary bulbs would probably not have glowed, but those little fluoro's dont need much energy to make them glow, as it is the gas inside that lights up, not the element, so your wiring may have allways been wrong, but you never noticed? or is an ordinary bulb doing it also? let us know.
thanks, alf.