This tingling can be felt on our aluminum 13" Macbook from 2008 and a 15" from 2011 if the Apple extension cord is not used on the power adapter.
The V shaped slide part of the extension cord connector should have metal contacts under the edge of the V. These contact the metal button / mushroom shaped pin in the adapter, which is electrical ground. Make sure your three prong extension cord actually has the ground connection from the wall plug to the power adapter. This page might help you find the ground pin on your plug: http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets
I believe the ability to feel the tingling is more related to the condition of the skin contacting the laptop than to humidity in the room. Dry and thicker skin might have different results, for example.
Even if the extension cord has a ground wire then the outlet you are using could be wired incorrectly. Maybe try grounded outlets in a different or newer house or office.
This isn't just an issue with Apple gear - metal bodied IBM Thinkpads do it if the rubber coating isn't too thick. And I've had it happen on metal faced Pioneer audio gear. It isn't static, it is often felt only when the hand is very lightly touching the metal and often only when the hand is moving - as if brushing off a crumb. Unplug the power connector from your Mac and the problem will likely disappear while on battery power. The prolonged tingling nature of the effect leads me to believe it is a stray current. This is not likely due to a wiring problem in the device but might be a mild inductive or capacitive interaction between the power adapter / circuit and the device housing.
The fact that the laptop housing is metal and should be grounded when connected to power suggests that it is designed correctly to safely route true static discharges from your fingers to ground without damaging your Mac. I've damaged plenty of PC gear by working on carpet in winter via painful static shocks to the monitor or keyboard / USB ports. But I've never damaged Apple gear this way.
So the solutions seem to be: Make sure you are using extension cords with an embedded ground wire and plugs. Make sure the wall outlet is grounded correctly and any surge protector is undamaged. If this can't be achieved, use an external keyboard when connected to power. A silicon keyboard cover will completely cover the Apple wired keyboard, if needed. And powering the Macbook from an Apple Cinema display might be worth a try.