Like what others have stated, the primary cause I'm afraid is the viewing angle. Remember that light refracts once it hits a kind of prism, similar to how we see the rainbow after the rain.
Mine has got this slight yellow tinting but when you change your angle of view, it disappears. This also answers the question why 27" ones are more prone to this than the 21" considering the viewing angle required for a larger screen.
Yes, viewing angle can cause some difference in appearance between the top and bottom of the screen. No, it is not the primary cause of the yellow tint issues reported on these forums.
Two tests if you think you are just blowing a viewing angle difference out of proportion:
1) Look at the screen from a distance. The further you are, the lesser the difference in angle between the top and bottom of the screen.
2) Flip your mac upside down and see if the "problem" flips upside down as well. If it doesn't, just viewing angle.
I did test 1 at the Apple store and verified that the yellow tint was real on the actual floor models (i.e., the difference in uniformity was visible from across the store).
I did test 2 on my own iMac, and the yellow tint that was "on the bottom" flipped when I flipped the screen. I returned it.
If you don't care whether your screen is uniform or have a perfect screen, that's fine, but then please don't try to tell us that we're just seeing things.
