I wonder when rumours says it will be described as "Ultra thin", if thats based on it being thinner than the current Air, or if thats what they define thin as with a "fatter" display with Retina compared to a thinner display that is not retina. If that made any sense.
To clear it up, they could just say "The thinnest notebook with a high resolution display" when they reveal it. I don't think Apple would make it even thinner as it is, but rather keeping it in the MacBook Pro Retina shape, without the thinner edges as thicker back, which will then be a much more thinner version of the MacBook Pro Retina.