The usual.
Retina can be done cheaply with a TN panel like on the Touch. Would they go that route? I doubt it. They've already established that nice screen and how nice it looks even if you and a friend are looking at it together.
And remember Retina on a bigger screen would mean much higher pixel count than on iPhone or Touch. The same resolution as iPHone or Touch wouldn't be the same pixel density and so they might even call it Retina.