According to an official Apple statement, the reason you won't see any white iPhones on shelves tomorrow is that they've "proven more challenging to manufacture than expected". Full statement:
CUPERTINO, Calif., June 23 /PRNewswire-FirstCall/ — 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.
The idea that one case colour would be more difficult to produce than another seems fairly implausible, but then again, Apple could just be referring to general supply line shortages as the umbrella idea of manufacturing challenges. Either way, it doesn't really matter. If you wanted a white iPhone, you'll either be waiting until mid July or mastering the fine art of white-out.
UPDATE: From a reader/Design Manager who knows a thing or two about mass manufacturing, white equipment may indeed be harder to produce than black:
"The process to make the back of the glass black is an electroplating process. This creates a very durable consistent and smooth black finish. To make white on the other hand you have to paint the back of the glass with white paint. It is difficult to create a smooth opaque look with the white. Also if the white paint goes on too thick it is difficult to glue the glass on because the glue will stick to the paint, but the paint will pull off of the glass. "