How does the quality of the LED-backlit screen of a MacBook or a 27" Cinema Display degrade over time?
This Engadget article shows photos of a direct comparison:


  Standing in front of the white-LED backlight on the new MBP, it's immediately apparent how crisp the whites are -- it's very noticeable after staring into CCFL LCDs all day long. But even though the whites were whiter, the screen didn't necessarily seem a lot brighter, which is probably where some amount of CCFL dimming on our test MBP came into play in creating the dramatic difference we saw. Viewing angles were very similar on both machines, but as you'll notice, at half brightness it's the LED backlit MBP that's dimmer, and the CCFL display that's actually brighter. Interesting stuff.


Also interesting could be the Advantages and Disadvantages sections of the Wikipedia article on LEDs (link):


  
  Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.
  Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000–2,000 hours.