500KeV discharging through ground through, say, the metal surround to a DVI port or similar can be just as damaging as discharges inside the case. This is actually more of a problem with the plastic-cased macs than with metal cases, because the charge has a time to build up and the only places to discharge are the ports/connectors.
It doesn't matter that the current is very low. This is why we take such careful static precautions when handling ESD devices, where as low as 5 eV discharges can be fatal to a component.
Someone also asked about using WD40 to clean the fan blades (in an IM to me). This is not recommended because the blade is left with a slightly sticky covering that traps dust, which is then much harder to remove. For the same reason, never oil a printer with a wet lubricant.