As others have said, that's not it. It's the line filter capacitors in the power supply. There's two types: X capacitors which filter differential noise (between line and neutral) and Y capacitors which filter common mode noise (between line and chassis.) Some small AC current (microamps) will pass through the Y capacitor to the chassis. If the chassis is ungrounded, you may be able to feel the current when you touch it.
I don't have a macbook so I can't comment on it, but I have a touhgbook with thinkpad power supply, and I feel it only on my wrists when I'm well grounded. Can't feel it on my fingers or palms.
