Mine does appear to be warmer at the top then the bottom, However, if I open a completely white, blank page, it looks rather uniform. I saw my buddies iPhone 5, and his is noticeably more yellow at the top. It's nowhere near gradual as I have seen on mine or with some of the pics in this thread. IPS displays(at least monitors) tend to have color shift or a warm glow to them if the display isn't perfectly perpendicular to the desk(or whatever solid flat surface is below it) and you aren't looking directly at it.
Personally, it's a non issue. Everything else is absolutely perfect. No dead/stuck pixels, no dust, screen is flush, nothing is washed out. I learned the hard way over the last few android phones and tablets. It can become an exchange nightmare and really does take a toll on the enjoyment of the device as a whole. To my great displeasure, I would sometimes end up giving up. The funny thing is that soon after I would cease to notice whatever "flaws" I noticed initially. I've just learned that no matter what, there is always something to gripe about. I'm thankful that I was able to get a near perfect device on its release.