Apple has always had a problem with getting both colors of the iPhone out at once - and the iPhone 4 is no exception.
Apple has officially pushed back a white-colored release of the iPhone 4 until spring 2011 - they won't say why, but there have supposedly been manufacturing issues from day one.
By the time spring 2011 comes around (Q2 2011) we might be looking at a 5th generation of iPhone... do you think the white iPhone 4 will even see the light of day? See what I did there? Light, day, white? Yeah, I went there.