It could just be that it was too expensive to redesign the case without the Firewire port. Apple would never leave an unused hole in the case or plug it with a blank (like some other manufacturers do).
Could they have simply used the motherboard from the MBP, which supports Firewire, but use the MB's chipset for CPU and graphics? Of course, I guess they'd still need to get the DisplayPort converted to mini-DVI. But that would allow them to update the white MB without creating a whole new motherboard or a new case.
Guess we'll juset have to wait until someone gets their hands on one and tears it apart.
I guess I don't really know how usable the MBP motherboard might be for this new white MB. Is there a significant motherboard change going from FW800 to FW400? Regardless, the fact that Apple did make a significant update to the white MB and kept Firewire seems to make the lack of FW on the aluminum MBs more of a head-scratcher (whatever the real reason for it's removal).