When was system grounding code enforced in the US. This is just for my personal curiosity.

Do you mean the grounding electrode system?
Or equipment grounding for branch circuits?

Im talking about when the electrical wire went from having just neutral and hot wires to having both of those plus a ground.

In 1971 the National Electrical Code required 3 prong outlets in all locations of the home, so that's when wiring of 3 conductors (or two conductors plus conduit for ground) became common. The refererence to the 1971 code is here: http://en.wikipedia.org/wiki/Cheater_plug

If I recall correctly, the 3 wire system that allowed the neutral and equipment ground to be one conductor, for appliance branch circuits and panels, was removed in the 2005 edition of the NEC.

Please explain why your asking.