An interesting point, Bobb.
Law 14 states that "the ball must be stationary on the penalty mark", which I see as different to those rules which require the ball to be within a specific area. So the flexibility in postioing the ball, which applies when taking a corner, doesn't seem to exist when a penalty kick is being taken.
BTW: I love the part of Law 14 which states "the goalposts, crossbar and goal net must not be moving". Is that in the laws, I wonder, to allow for games being played in a hurricane? ;-)