I've seen some online threads and at least one StackExchange question on this, but the answers don't seem to apply to my situation.
I have a 2009-vintage white 13" MacBook that's always been in great shape except for needing the relatively common bottom case replacement a while back. I did the replacement myself without apparent incident.
Thereafter, though, I noticed that the exposed metal screws on the bottom of the case can burn/tingle my exposed skin if the machine is plugged in. And I do use the three-prong grounded (earthed) power cable.
I actually first noticed this while charging my 2nd-generation iPod Touch from the USB port while the laptop was charging; the whole back of the iPod had a live feel to it -- a little stuttery resistance as you passed your finger over the metal, which went back to being perfectly smooth as soon as I unplugged it. And I also have a pair of earbuds with metal casings that are downright painful to wear when plugged in to the laptop.
All these things only happen when the laptop is plugged in. Remove it from "mains power", as the non-Americans would say, and all these effects cease immediately. Happens with two different power cables I've tried, and in all different locations. (Well, given that all those locations are homes and offices with standard U.S. wiring.)
I've tried retightening the bottom case screws, but they're as tight as I can comfortably make them without worrying about stripping the head or the threads.
So -- anyone have any new insights on this one? Thanks!