Its likely the 12GB card is for a Quadro version - with the original Titan card,the Quadro/Tesla versions were shipping for months before to high level customers. In this case,we have not heard a thing,which makes me think that he Quadro/Tesla versions will launch at the same time as the consumer Titan version.
So far as 12gb of ram goes in a card, there is already a 12gb Quadro card being the Quadro K6000 and it has been out for quite a while now. There is a real chance that that the Titan II could go up to 12gb of video ram.