On my iBook, the screen was noticeably dimmer after ~4 years. I mean, I didn't really notice it until I had my laptop next to one that just had the screen replaced. Maybe it was just dim to start and the replacement was brighter than OEM. It's a slow process and it's something you just get used to as it happens, but it's not something I'd worry much about.
If you have the LED backlit model the backlight life should be considerably longer. If you have a white/blackbook that's not the new Unibody type, yours is CCFL.