Its often difficult to find this type of information. If this info is avaiable for this product, I never saw it either. Typically however, it might indicate major or minor differences in the internal hardware. ie: chips used on the controller board, power supply or antenna type. Vague answer I know, but I looked too and never came up with anything.
I picked up the B version for no other reason, except that I noticed more recent firmware availability. I have to hand it to DLink for putting the hardware Rev and firmware on the outside of their boxes. You rarely see this elsewhere.