IEC/UL 60950-1 allows for up to 3.5mA of leakage current to be "accessible" to the user. There are a lot of variations and factors that affect how you feel the sensation of current through your body (how moist your skin is matters a lot), but even microamps can be painful in the right situation.

I love a lot of things about macbooks, but I don't love the non-coated metal for this reason. In addition to the (for me) unpleasant feeling of current, the bottom feels hotter than a coated surface.