As for what the challenge may be, we have no idea. Is it tough to make perfectly white material for that back panel? It could just be, of course, that Apple had some unforeseeable problem on a certain production line, leaving the white models in the lurch.
At any rate, Apple says that the "more popular" black models are good to go, so those will be ready in the stores tomorrow. But of course if Apple has only half of the models in stock, that too could affect availability. We'll have to see how things go tomorrow morning.
[hat tip to MacRumors]