When a lamp socket is installed permanent the white wire goes to the shell and the black wire to contact in the center This is so it is harder to get a shock. (The white wire and shell should be at zero volts to ground.) If you want to be nice you could find a polarized plug. example http://www.stayonline.com/detail.aspx?ID=4733
ot put on a 3 prong grounded plug. This will force the correct polarity.
BTW White is the wide blade is the silver colored screw. black is the narrow blade is the brass colored screw.