basically, without a ground wire in the wall connected to the outlet, if your equipment were to short to the casing there would NOT be a complete path to ground for the electricity. this would result in the casing being as the same voltage as whatever was faulted (ex. a power supply cord shorted to case=120VAC) so if you went to touch it you get painfully tingly. also it could present a fire hazard if something flammable was touching it. now with a grounding wire installed in the plug, the flow path of electricity would be from the fault to the case, through the ground wire in the power cord (the bottom center, round conductor) through the ground wire in the outlet, to the house's common ground, and then literally to the ground. i can come up with a picture when i get home if you'd like, i'm at work now. you know, i'm glad this topic came up. i finally get to use some of my 6 years of electrical experience in the Navy!
