This is actually a very common problem with computers. I built a PC a few years back, and whenever I touched the case, I would get this weird feeling like electricity just flowing into my arm. It was really really scary. Eventually, I discovered that when I was attaching the motherboard to the case, I dropped a screw behind the motherboard and it was allowing electricity to flow from the motherboard into the metal of the case. That also explained stability problems I was having. It is possible that something inside the computer is not grounded, and electricity is making contact on the metal case. Just take it into Apple and they will fix it. It will probably involve taking the entire thing apart though. You might want to just request a new one.