We got two iPhone fives, one white 16 and one black 32 and a black one had a noticeable light leak issue on top left corner.
It wasn't noticeable at first but when using Apps that didn't take up the full-screen or dark content movies you did see it for sure.
I called Apple and they sent me a replacement by mail through their express service. The new one that I just got has a perfect screen so it's a keeper. I'm sending them the old one back tomorrow. So at least from my little experience one out of three had this issue.
For what it's worth the replacement I got has a build week number of 44 which is November and the old one that I originally purchased (With light leak issue) was a week 48 which is December build.