LCDs aren't like CRTs.. you don't have to worry about the backlight burning out like the old tubes did. The screen really can't go bright enough that it will reduce it's usage by any significant time. I mean, things can go wrong, but it won't be by keeping the brightness cranked up.
Untrue. Most LCDs use CCFL tubes, which are like a very tiny fluorescent light bulb - and just as with fluorescents, they do eventually burn out and also fade in brightness over time. Now, the rated life span of CCFL tubes is something like 20,000 hours+, so the chances of it burning out during the useful life of your laptop are pretty slim, but it does happen.
The newer MacBook Pros are now using LED backlights, which are as yet untested since they haven't been around all that long. But, seeing as how LEDs get dimmer over time, I'd imagine the same will hold true for them.
But the question at hand is whether running at full brightness will shorten the lifespan of the backlight or cause it to become dimmer faster. My opinion is that's unlikely to happen during the useful life of your notebook (3-4 years), regardless of what brightness you run it at - and if it does, it's not a terribly complicated process to replace the screen.
