"Good post but not entirely correct. You don't have to be using a three prong device to be protected by a GFCI receptacle.
The GFCI will trip if it see's a current imbalance between the hot and common or if it see's current from hot but NOT on common, which suggests the current is taking another route - through you."
Correct. In fact you can replace standard non grounded outlets with GFI 3 wire outlets and be up to Code. Frankly I would like to see a ground wire or Earth connection, but sometimes in old houses its just about impossible.