I used to get tingling from my 12" Powerbook too. It's only really noticeable for me when I'm in China on 220V. In the US, it's barely noticeable. Once, I was touching my girlfriend's stomach while she was using my computer and I was getting a little buzz from her skin!
Maybe it's just an issue with the ungrounded 2-plug brick and the aluminum case. With the 3-plug extension cord, there's nothing.
Thank you for this, using the 3-plug extension cord solved this for me