A design overhaul wouldn't require the dev centre to go down at all - they could just prepare all the assets etc on testing servers and switch them over when they are ready.
Given the normal warning given on any maintenance, and the obvious negative consequences for Apple's business of any extended outage (extended in this case being over a day or so), this is unintended, and most likely caused by a security breach. I think we can safely rule out a design overhaul unless Apple are incredibly incompetent.
NB itunesconnect is still up (the bit which deals with app upload, itunes store metadata etc), only the dev center - dealing with certs/device registration/distribution etc - is down.