That's kind of my point, loud speakers have 'standards' while headphones/earbuds appear to have none. You can buy them with 16ohm all the way up to 350ohm(I think is the highest I have seen). A standard for this item should almost be mandatory considering how close these devices are to your ear drum.
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.