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.
Frank