Apple has missed the target date for the white iPhone 4 twice now, so you'll forgive me it I'm a bit skeptical of the newest target date of "spring 2011." This information isn't new. An Apple spokesperson also reported that the white iPhone 4 launch was rescheduled to spring 2011.
The unofficial reasons that I've seen around the Internet range from the white parts of the phone not staying white and yellowing to there being problems to the cameras to an antenna redesign. Whatever the reason, the end result is the same - customers still can't buy a legit white iPhone 4.