My Model S was slated for delivery in late October / early November, and I also noticed my delivery status changed to late December / January. I am betting they have enough inventory to get through the next month or two, but then the supply for the gray next generation seats is gone.
Tesla can't afford any shortage that could delay deliveries or compromise sales until January, so I bet we will be offered a change to a different color for no extra fee or we will be hearing about new seat options in the near future!
C'mon Tesla -- give us ultra white ventilated seats!!