I thought our money already had that. There is a coded line
accross the bills that you can see. Many stores scan this with
some kind of apparatus.
As for Gold coins, they are easier to do that bills. I remember
in the late 50's, near Chicago, getting lead dimes and quarters
in change. I certainly couldn't see any difference, they just
didn't bounce very well. Modern Hobby milling machines
coupled with computers, should make duplicating coins
a fairly easy task. Plating is easy too.
They are working on it now and it may be out in two years. It will be for large denomination notes first. Posted an article on it before Christmas but can't find it.