I have this on mine (which, incidentally I am taking to get swapped tomorrow due to yellow tinge and big hole in the backlight seal at one edge!).
I would describe the sensation as similar to the feeling of 'ringing' a wine glass, it's a kind of subtle vibration. I don't think it's electrical, just an interaction between your fingertips and the texture of the aluminium surface. I've had this on a unibody Macbook at work too.
Oh and @WozzaTT - LMAO!
