So; Steam. Digital distribution. No physical media anywhere.
One could assume that Valve essentially has an unlimited supply of everything on Steam. If they need more they just generate more. I don't think that's an entirely unreasonable assumption since it's all digital. They gotta be able to generate keys, right? You buy something and it automatically makes you a key and everyone gets paid.
Not so. Apparently they do have a limited inventory and apparently it's entirely possible for it to run out with them unable to "restock" fast enough to meet the demand. At least, for one game so far: PREY.
Perhaps PREY is a special situation. Regardless, it's an amusing situation. A game sold out on Steam. :P
Anyways, that just made me laugh. :)