Classically, White has been consumer grade and black has been the professional grade in Apple design. See iPhoto vs Aperture, or iMovie vs Final Cut Pro. The Macbooks were all white, while the pros were (admittedly not black, but) metallic. I don't think it is that much of a change from what they've already projected out there.
This rule was left behind some time ago. Eg, Macbooks are also silver - even the Air. Obviously Iphones are both black and white, regardless of features. But who knows, maybe the new tower is a throwback to the old days?
