I have been following LED developments pretty closely for some time now. Got really interested in LED flashlights, and have a few 1 Watt ones, waiting for some 6.5 Watt Luxeon K2 upgrades for my several Maglite 3 cell lights. Luxeon is way behind their release dates for the modules.
There is one problem with them that will have to be addressed for headlights though, and that is the very narrow wavelength band of the light. I am following LED flashlight developments on a website started by flashlight hobbyists (you won't believe what some of these people have tied up moneywise in these lights!!). I was amazed at how fanatical some of these people are, rivaling our LT enthusiasm.
The issue with the narrow wavelength is outside use, as depth perception is lousy. Incandescant lights put out light in a pretty wide range, and colors and depth perception are good, but narrow band wavelength emitted by current LEDS in the white range cause a lot of washout in colors, and drastically reduced depth perception, causing a scene to look "flattened". LED flashlights are great for indoors and task lights, but not so good for outdoors use.
It is just a matter of time though, as white LEDs are still in their infancy. The LEDs themselves put out very blue or UV light, which is turned into yellow by exciting a phosphor coating on the surface of it. I imagine the development of these coatings is where the advances will be, hopefully getting them to emit a wider color range. If you look at the LEDs in current white flashlights, it looks yellow. That is the coating.