Says Apple: "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."
According to the Chinese newspaper 21st Century Business Herald, the delay comes from manufacturing problems in a Chinese factory with the company Lens Technology, which transforms raw glass into iPhone 4 glass panels.
Apple has never confirmed what is causing the delays on the white models.