I have taken a solid state electronic devices class here at U of I (one of the hardest classes I have taken), and the wave-length of light emmited by the LED depends on the energy band gap of the semiconductor. By mixing different semi-conductors together, and by having them in different lattice patterns, virtually any bandgap energy is possible. There has been a lot of research in this area lately, and I definately see it as being the future of lighting. The biggest cost is the making of the semiconductor, since it is not usually silicon, there are not huge factories dedicated to making the correct mixtures in the right crystal form. I just wish I could find a way to make them cheaper.
-James