MacBook Pro 17" (A1297) - Keyboard backlight dimmer than when first purchased
White LEDs, as used in the keyboard backlights in apple laptops, do dim over time.

White LEDs use a blue LED, and an phosphor which is excited by the blue light, and re-radiates the energy in the orange-yellow spectrum. Unfortunately the phosphor does decay over time, reducing the efficiency of the LEDs.

However, the lifetime of modern LEDs is typically something in the range of ~50,000-100,000 hours to 50% brightness, so it's likely that there is some other mechanism in play for your situation. There are a number of external factors that can effect LED brightness, from poor-quality phosphors to overdriving of the LED die.

Can you estimate the brightness difference? If it's only a few percent different, it may just be normal aging, or lot-to-lot LED variance. 

Anyways, if it bothers you, it may be covered under the apple-care warranty. It might be useful to take it to an apple-store and have them look it over.



Here is an interesting white-paper on LED life expectancy and ageing effects.