Well, LCDs do go down in brightness over the course of time as the compact fluorescent that provide the illumination get weaker, this is one reason that the LED backlit panels command a premium, they will run for tens of thousands of hours with little to no reduction in brightness.
You would have to have it on an awful lot though to have a noticeable drop in brightness after just two years.
It is possible that Apple Care would cover this, you should call Apple about it.