Grey tint at top of screen on iPhone 4

Hey, recently received my iPhone 4 and noticed that on a bright screen, white or light background, there is a definite gradations of grey coming from the top of the screen when looking at it in portrait.
I'm going to swing by the apple store and have it checked out. But before I do that I was wondering if anyone else has experienced this and if it is due to the backlight or something I should be worried about?
Thanks in advance.