I think thats the one of the bubbles from the background image.
Retina display is working!
lol yeah could be screen working, there must be a white hair of some sort that just bout matches up with the bubble as it extends over the black non-pixelated are at the bottom of it.
That black non pixilated area doesn't show up as much on a black iPhone but would seem like a secound black border as would show up more on the white one.
I like the look of the white one and the only thing that bothers me is the viewing experiance for me the user. Aint bothered as much about fingerprints as will prob get a leather/felt lined Seana case again for mine which seems to clean it everytime i put my phone back into it's case.