This is an interesting thread. The technique used by the classical players is slightly different because they play a harmonic by both fingering the fret with their index finger and plucking the string above it with their little finger WHILST holding down a shape with their left hand further down the neck. It's really nice but it's not tapping.

As much as I loathe the whole package Eddie Van Halen was the first to do it and I give a grudging respect to something that was genuinely new. I also hate the shredding thing but do go along with the view that these fellas know their guitars really, really well and practising scales through an amplifier it certainly isn't.

By the way, Allan Holdsworth can do with one hand what Van Halen does with two.