Physics One: Shocking Results

Luboš Motl gives us a nice physics problem:

Yesterday, I experienced something that I've gone through many times in my life and probably regularly (at several "best points"): when I ride my mountain bike which has a hole in the seat and a metal right underneath (which may probably be in contact with my slightly wet shorts), I am terrified by an electric shock – some tingling comparable to a dozen of stinging ants attacking a square decimeter of my skin – for a second when I am crossing some high voltage power lines – maybe 400 kV or so – perpendicularly.

It seems clear that passage under the power lines has induced a charge separation between bike and rider, and that the electric fields induced by the power lines create the charge separation, but what is the mechanism?

Ideas? Can the bike be considered grounded?

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