In an interview with Ina Fried at AllThingsD, Phil Schiller and Steve Jobs gave a little insight into what made the white iPhone 4 so tough to produce. Actually, it wasn't really the manufacturing itself, but the materials science needed to make sure the white iPhone was in every way as capable and durable as the black-painted iPhone 4.
Schiller told Fried, "...There's a lot more that goes into both the material science of it – how it holds up over time...but also in how it all works with the sensors." There's also a reference to how the white iPhone needs more "UV protection" than the black one, which I would guess comes from the light that is not absorbed by the black paint passing through the white paint instead.
There's a lot of tech that goes into the iPhone, and I'm really not surprised that something like a coating of white paint turned out to be tricky, but I think everyone (including Apple, it sounds like) was surprised at how long it took to sort this out. Still, the white iPhone goes on sale tomorrow. Will you get one? Will you apply sunscreen to it?