That's why I don't think you see really high current current headphones like 4 ohm. This is just a guess but portable MP3 players are already designed for lower impedance headphones (16ohm-32 ohm). My guess why you don't see 4ohm or 8ohm headphones (typical sensitivity) is they would be much too efficient considering the sound source is right against your ears. My hypothesis is there would be no room for volume adjustment. It would go from soft to REALLY loud too quickly.
I don't think he's suggesting 4 or 8 ohm as a standard for headphones, just that headphones should have their own standard, so there's not such a huge range of possibilities.
