I'm surprised this many of you are mentioning this happens all the time or that you're in the habit of touching a body panel when exiting.
I thought it might be more related to your geographic location but you guys seem to be scattered around a bit.
For the record, I've been driving for 13 years, I've personally owned 7-8 modern cars in that time and I have never, ever been zapped by any of those cars. Nor can I remember being zapped as a kid prior to owning my own car.
I have had this happen on my '65 & '63 Caddy's. I attributed it mainly to bad grounding or something. It seems to happen all the time on my '63. Again, has never happened to me on a modern car as far as my 31 years of memory serve.
Are you guys rubbing your feet on the carpet floor before exiting to build up some static charge or what?