The elaborate test involving the puddle was absolutely correct, but I woudn't try it myself.
Arnel's comment about the two different plugs is on the right track--assuming you're in the US, the two-blade plug that snaps directly onto the square block is not grounded, while the longer cord with the three pins is.
However, although I can't test this, I think the ground pin is only for shorts in the power brick; the thin DC cord running to the powerbook doesn't seem to have a seperate ground line, so unless the problem is in the brick, switching probably won't tell you anything.
Ok, here's some advice (really repeating what several other people already said) that should tell you something:
If it's your PowerBook that's building up a charge (which sounds a bit odd to me, but I suppose it's possible), then you can do this to test: ground the PB's metal case. Just run a metal wire of some sort from the PB to something definitely grounded (a metal part of your house would be the easiest and safest, or you can use the ground pin hole on an outlet if you're careful/know what you're doing/brave). A static wrist strap would be an easy way to make the connection.
If that fixes your problem, then it's the PB with the charge, since the ground connection bled the charge off and left you safe.
If you still get shocked, then it's almost certainly you with the static charge.
You could do the same thing by grounding yourself, with revesed results--if you get shocked when you're grounded, your PB is the problem, while if you're fine while grounded it was static on your body.
The easiest way to ground yourself (but slightly less positive) would be to touch a doorknob or something similar and metal, but a wrist grounding strap would be even better.
I'd say the grounding yourself test is slightly less positive than grounding the computer case, but either would probably give you an idea of the problem.
