After putting the floor down I realized I needed some more light to work with, so I put up the Durrock and recessed lighting fixture that will go in the ceiling of the shower.  The fixture is an airtight can, since the other side of the ceiling is the attic, this will prevent any air leaks, and the cover for it is a gasketed, shower specific cover, that will prevent the bulb from getting wet.   The installation actually went pretty easily, once I had the Durock cut to size, I hoisted it up, and with the help of my wife screwed it to the joists.  Then I climbed into the attic and was able to slip the hangers for the light underneath the joists, and adjust them so that the can was centered over the hole.  This was alot easier than putting the light up first, and then trying to get the hole in the Durock to match up.