The lifetime rating for LEDs is typically to 80% of initial brightness level. The LED doesn't blink out instantly, its brightness gradually degreades over its lifetime. Lumileds usually shows between 70 and 80% of initial brightness on their lifetime/brighness graphs at the endpoint of 100K hours (this is for 1W and colored 5W luxeons). Degridation of LED brightness begins immediately, just that the rate of degridation is impreceptable (especially for those rated for a 100K lifetime)
Overcurrent can accelerate the degridation, as well as high junction temperatures (less effective cooling).
Since the colored 5W LEDs still retain their 100K lifetime, it is logical to conclude that the 5W white's low lifetime is due to the degridation of the yellow phosphor (as white LEDs are a blue LED covered in a yellow phosphor material). In reality, after the 1000 hour rating of present 5W whites has expired, you could still expect that the blue LED die beneath the phosphor is still at around 90% of initial brightness, just that the phosphor material isn't "phosphoring", and is blocking much of the light.
The phosphor degridation may be a result of higher temperatures (likely) or breakdown of the chemical due to the release of more photons (just a guess).
