Apple has just released a press release stating that the white iPhone will not be available for purchase until the second half of july because "it is more difficult to manufacture," but nobody knows what those difficulties are. Apple also says that it will not affect the supply of the black iPhone 4. So if you want an iPhone 4 now, you have to get the black one, but if your heart is set on the white one, well another month you will have to wait.
Here is what the statment said:
"White models of Apple's new iPhone® 4 have proven more challenging to manufacture than expected, and as a result they will not be available until the second half of July. The availability of the more popular iPhone 4 black models is not affected."