most night lights are designed to attach to the seatpost. but while carrying a bag on the rear rack that area get's blocked.
some manufacturers sell cheap, awful, lights that attach to the rack (topeak) but i figure some kind of adapter would be much more useful (mostly since everyone may have night lights before getting a rack... i do :)
but my searches for such a adapter failed. do you know any?
This is one example of night light i have http://www.dealextreme.com/p/rechargeable-7-mode-red-5-led-safety-bike-tail-light-with-7-mode-2-red-laser-beam-35mw-90117 (i regret. do not buy, laser is useless snake oil. but the leds are OK)
and this a rack http://www.blackburndesign.com/racks/mtn-2-rack.html (it has a hole on the back, where you can fit a L shaped adapter that holds a reflexive cat's eye)