The vertical viewing angle of the MacBook display is very narrow. When you are looking down at the screen (ex. in your lap), the bottom of the display will appear lighter than the top. If you put the MB on a table/desk and get it closer to eye level, the difference in brightness will be minimal.
This bothered me about my MB when I first got it, but I do not believe it to be a defect. If you think yours is defective, compare it to other MBs at a store.