"I wonder if the original American plug was intended mainly for Line (120V) + Line (120V) + Neutral, thus making it a 240V plug?"

I've been wondering the same thing...if an Australian appliance was plugged into the equivalent 3 pin socket in the US, would the casing be connected to earth or neutral? Is it used for a straight single phase 240V supply with earth, or is it a 120V two phase supply with neutral on the 3rd pin? The appliance would obviously work either way, but I'd feel safer if the case was connected to earth and not neutral. The US method of wiring appears to use something like our M.E.N system so the neutral and earth would be at virtually the same potential.