I don't know if it's Lion or a hardware feature, but on my MacBook Air, the system changes the keyboard lighting (and screen brightness) depending on the ambient brightness, which is detected by a sensor near the iSight lens at the top of the screen. In bright surroundings, the keyboard lighting is disabled and pressing F5 or F6 does nothing.
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.
Best Answer
It appears this warning icon means that it's too light to turn on the Keyboard Backlight. Very misleading icon suggests an error.
You can test this by holding a flashlight up to the light sensor.