You can find several anecdotes on Mac/Apple forums and they claim it's a known defect. On others it's a green tint on the edged not yellow, others get a pink central area, and what's weird is that apparently it's visible but the phone owner doesn't notice it. Usually it's the husbands, brothers, friends who notice it, until you put it side by side with another iPhone. Defect occurrence range from within 6mo to 2yr, and as far as I can tell goes as far back as the iPhone 4. Guys at AppleCare were quoted to have said it's a common issue.
I guess it's worrying from an LG standpoint sincr LG provided like half the screens Apple uses.