That's a really good question that I have never
thought to ask. I'm assuming you're talking about
the 3-prong outlets which fit plugs with 2 flat
blades (on 'top') and one round pin (on 'bottom').
The two blades carry the current, and the
pin is the 'ground'. The ground pin only
carries current when there is a fault in
the system. A fault is when something disrupts the
usual flow of electricity through the circuit --
if one of the wires gets damaged, or something
metal touches an exposed wire. In situations like
that, the ground pin will carry the extra current
away from the fault and to the ground (literally)
which should prevent all the current from going
into the fork that you're holding. This is why
the 3-prong plugs are safer than the 2-prong
variety, and also why you tend to see them in
devices that are used outside, or around water, or
in hospitals, which are all places where some
moisture is likely to get into the device, causing
a fault, and potentially giving a nasty shock to
anyone who happens to be touching the device.
So, why put the plugs upside down?
The best answer I could
find was on a message board for electricians,
where one contractor suggests putting the ground
pin facing up for this reason: if the plug isn't
all the way into the wall (so that some of the
metal parts of the plug are exposed) and you
happen to drop something metal onto the exposed
parts of the plug, it would be best for the
dropped object to contact the ground pin
(causing no damage) than the current-carrying
blades (potentially causing a short-circuit).
I suppose they don't want to take any chances
in the hospital! So remember to always be cautious
around electrical appliances and wires. Also, even
3-prong plugs won't always save you, you can still
get some rather nasty shocks just from touching
the insulated plastic around a 3-prong outlet.