Your comment reminded me that as a kid (3-4 years old and even a little older) I used to, in a sort, anthropomorphise a power plug which I used to find very nice-looking and of course that I would play with it. It was driving my dad crazy, for good reasons (I actually did manage to get electrocuted at 3 years old while playing with another plug), but somehow I survived. All this to say, not so tongue in cheek (or only partly), that we should maybe strive to make angry and ugly plugs as much as possible.
The convention, now, seems to be to put the ground pin on top. I am not entirely sure when that became the convention, however, as it seemed to be the other way around when I was a child (with the ground pin on the bottom). I think I was a teenager when I started seeing the sockets installed "upside down."
EDIT: Upon googling, I can find lots of conflicting opinions, but it appears that the electrical code has nothing to say about it. They can be installed in any orientation: ground up, down, or sideways.
Also, wall warts are also required to have all of their weight above the plug in the ground on top orientation. That's never worked well for me.
EDIT:
Is it realy a convention?
I was told so, but after googling, I am beginning to doubt it. It may have been a short lived thing.