In all other civilized and not so civilized countries in the world (I've visited 17 and lived/worked in 7) the groung pin is ALWAYS at the top. Why? for safety reasons. No matter how the plug pulls out, the ground pin is always last to leave the receptacle, they are designed to do just that.
The main reason though, is that if a plug is partially pulled, and a metal object falls or accidentally finds it's way behind the plug (usually from the top, gravity you know) it will touch the ground pin, and not the live & neutral.
That's the real skinny, folks. Ask the designers at Leviton or G.E.