I have an ipod touch fourth generation; the buttons on it just randomly died. So I called applecare and explained the problem. I also mentioned light leaks, which I was unhappy about but ignored.
Anyway, they put me on the phone with someone higher in technical support, who explained to me that light leaks are generally caused by the assembly of the ipod itself being "off", specifically, how the glass unevenly meets the case and needs adjustment. If the ipod is sent to applecare for light leaks instead of instantly exchanged (which he conceded most stores can do, but mine couldn't because it was engraved), then the ipod will get fixed.
I verified that this is probably true; the right edge of my ipod was sort of like sharp metal sticking out; I always though it was so your finger could trace the border of the glass, but then I noticed there was no "rim" effect on the left side of my ipod (where there were no light leaks).
So it was off by a few nanometers.
Anyway, the staff said do not ignore light leaks, because they're caused by the glass being installed improperly, and if not fixed, dirt and moisture can get into the ipod, breaking it. And it seemed, instead of swapping them out, best is to call Applecare, explain the problem, and ask for them to adjust the glass. Most representatives who aren't high up on the technical support totem pole do not know what light leaks are or what causes them on the current ipod touch 4. And again, its important to fix it.