In the August 2009 issue of IEEE Spectrum, Richard Stevenson writes about a problem specific to LEDs known as droop.
Blue LEDs are those used to produce white light. Once the diode’s power level reaches an output sufficient for general lighting applications, the efficiency of the LED declines significantly- requiring more power to maintain a usable light level. Until the scientists and engineers understand this problem it may be difficult to overcome- at least in the near term.
The LED’s Dark Secret has a detailed explanation on LED Droop. I found the graphs helpful and the article also helps explain how LEDs work.