The 6mm rule is very sound and should be observed everywhere -- it's amazing how easy it is to go from "tight" to "touching" -- esp. with regard to electrolyics, whose top plates are at full potential. (Says a man who has rested his fingers on the tops of a few too many caps at 350 volt potential -- that wakes you up quick!)
3-pin grounds are a good idea, but you often only want one of them per system (usually the highest-current component). You can use one of those fancy hospital-grade plugs if you want, though these are probably only valuable if you have many hundreds of watts of poer (though I've used them just because they look neat). This approach is no less safe (as long as the chassis of all your components are connected to signal ground), and it observes good grounding practice (i.e. single point).
Things aren't as bad in California as you may think. My building there had 3-wire earth grounds, and it was a mid-'80s postmodern stucco Melrose Place horror. I think most modern building codes in the more advanced states require proper grounds (though nothing as robust as you see here in Old Europe, where massive earth cables are the norm)... I do have a house in Canada that's entirely 2-wire knob-and-tube asbestos-insluated wiring from 1905 above the second floor. I had an audio system on its top floor for a while, and I strapped the earth ground (from the amp) to the copper water pipe, which is earth-grounded (and is lower impedance than any wire). Later I had a proper 10-guage circuit wired up there, not just for audio but to keep my son from frying himself.
Keep in mind that a lot of places just don't have earth ground. From my experience this includes not only most of the United States, houses in Canada built before the 1940s, large parts of France, most of Italy, huge sections of the Middle East, most parts of Africa, Russia, central Asia, and a good number of places in the Far East. I don't know about Australia or Japan.
The point is that if you really want to make something internationally useful, it's best to design it with the assumption that there won't be a functioning earth ground. Build it so that it will be safe without one.
In other words, plastic chassis rule (actually, I'm a huge fan of clear acrylic cases these days, both for safety and beauty -- and almost every major city has Perspex fabrication shops that will build you a nice thick shiny one for almost nothing).