I don't have a Mac Book, but I do have a "figure 8" power cord from some other device (2 lines, no earth). I still have the same readings -> drifting AC voltage on the iPad's metal shell.
My guess is as follows: Any exposed metal of a device should be properly grounded. The iPad's shell is probably connected to the ground connection of the power supply (i.e. exposed metal part of iPad connector). It is also common practice to connect the negative line of a DC circuit, which is otherwise free to float around, to ground. Now, if the power supply does not provide a proper grounding, you end up with the exposed metal being connected to the negative DC line which is free to float around compared to earth. When you touch the iPad's shell, standing barefooted on a stone floor (or being earthed otherwise), you are actually yourself grounding the device, conducting a tiny current from the device to earth.
In short: If using the provided equipment, as you should, the iPad's body is not grounded but connected to internal circuits, as it should not!
This is bad design, not poor, bad. If something goes wrong with the power supply (i.e. internal short circuit between AC and DC circuits) you could end up with an exposed metal body delivering full AC.
If no proper grounding can be guaranteed, the shell should be adequately isolated instead of being connected to internal circuits. Never should a person be able to come in contact with exposed electrics. The goal of the wall socket's earth pin is exactly that: ensure that any exposed metal of connected appliances will not pass on any current.
Again, this is just a (fair) guess. If Apple would have shipped a proper grounded power supply, there would be no problem.
I would appreciate it if anybody could confirm my guesses, as I have no intention of disassembling my iPad nor my power supply.
Message was edited by: Lexie$