Little suspicious. Think they're doing this so they don't have to give a free bumper with the white phones? Hold it till after August..... Interesting. APPLE!
This is an absolutely ridiculous and petty theory.
I think that Apple was unable to solve the antenna and proximity sensor problems in the white phone and made the only decision that makes sense: hold it off and avoid a new wave of complaints and media attacks that would hit with the release of the white phone. This the only sane and responsible thing to do, IMO.
Yes, I suppose now all those who wanted a white iPhone will cry and complain and childishly yell "I want it and I want now, and I want it fixed!!! And I don't care if it's impossible!". But it won't last.
On the other hand, there's a chance that Apple will do better testing on the phone and when it's finally released it will be free of the black one's issues. At least, I hope so. I wanted the white one too.