Apple’s statement in its entirety:
White models of Apple’s new iPhone® 4 have continued to be more challenging to manufacture than we originally expected, and as a result they will not be available until later this year. The availability of the more popular iPhone 4 black models is not affected.
From what we have heard, it has something to do with getting the paint level on the white handset to a standard that Apple is happy with. According to MacStories, the manufacturers are still working on getting the right balance between paint thickness and opacity. One to allow for enough clearance for the digitizer overlay and the other to reflect the high quality finish that Apple designers like Jony Ive strive for.
The news will disappoint some but the white iPhone 4 is a very sexy looking product. You wouldn’t want the digitizer to stop functioning as it should or the brilliant white finish to turn grey……would you?!