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.

To be pedantic, aluminum corrodes very rapidly in air, but the surface rust (aluminum oxide) is stable and shields the underlying metal. Aluminum oxide mixed with whatever dye Apple used, on the other hand…

Forming an oxide layer isn't corrosion. Corrosion results in loss of material like the pitting you see in that photo. You are correct that the oxide layer prevents corrosion, though.

Touché; I've been out-pedanted!

Residue from the manufacturing?

Almost anything corrodes very easily under human skin contact. If you take a pH testing kit, and put a drop on your skin, you'll notice you're pushing fairly acidic levels.

I destroy guitar strings on a near-weekly basis, despite all the anti-corrosion plating tech they have.

No, annodized aluminum is very resistant to corrosion. Guitar strings are made out of steel, which rusts readily and plating doesn't adhere as well to flexible materials like wires.

You use an acid flux to remove the oxidation on aluminum. Guess what your sweat will do if it is acidic enough? Hobo soldering tip 101 - no flux, use your sweat if you can get away with sodium in your solder and your solder has no flux.

My phone was in an Apple Smart Battery Case the whole time it turned bad, no skin contact.