Trying to power a metal-shelled computer with a 2-prong plug always seemed to me like an example of Apple painting themselves into a corner. The only two options as far as I know are to leave the case floating (which seems bad) or to tie it to the outlet's neutral pin (less bad, but still not great). The latter presumably leaves you at the mercy of how well the building's neutral pins are grounded. The fact that Apple decided to use fairly puny wires between the power brick and the computer probably doesn't help either.<p>[Also, the link on electrovibration seems to imply it's a capacitive effect, so it's caused by an electric (not magnetic) field.]