I guess they´ll ship them and later reveal them all at once (at least that´s how I would be doing it).
There was one instagram pic showing a white X in a store setting posted here, but the picture was removed from instagram later. From the info on the thread it was not a real store but a mock-up in the Palo-Alto HQ and an employee posted it.
Rep at the Scottsdale AZ showroom said they were told that they would get a showroom X in January. They have not dispensed much information about the X when prodded, I so I will take the January remark with a grain of salt. Hope it happens, but my expectations are low.
when I talked to the rep, I specifically asked if they had a sample of the white synthetic leather. He said they did. I am not sure if this is true in all showrooms so you should call ahead to see if yours does.