The prevailing theory is that it's a manufacturing defect in a small number of early devices (mostly ones shipped around launch day) related to some flaw in the anodization process. Since it hasn't happened to enough devices to warrant a class action, it doesn't make it on Apple's radar so it's just "cosmetic damage". In some cases Apple Store/Authorized Retailer staff has blamed it on third-party cases or protectors, but I had mine in an Apple case.