From what I've witnessed, I believe it's visible on a white background at any angles other than directly at the screen, lol. The original MacBook Air had almost the opposite issue, that is, a white bar of light at the top of the screen. That, too, was visible mainly at odd viewing angles. Don't have an accurate explanation for why it's there, but with these being ridiculously thin TN panels, I guess the manufacturers don't sweat the display not looking perfect at every viewing angle.
I have to say though, it's MUCH more tolerable than having backlight bleeding on a black screen, which I've found MacBook Airs to be pretty good about.