Apple will never make something with a "slide-out" keyboard. It's just bad design and Apple doesn't do bad design.
Nothing's impossible but a small touch tablet doesn't have the resolution to run OS-X.
The whole point of the iPhone GUI is that it is optimised for touch. Even such tricks (described in Apple's patents), as making the min/max buttons on OS-X desktop windows automatically get bigger when your finger gets close to the screen, are clunky adaptations at best and hugely complicate the interface. What's the advantage of sticking with a familiar (desktop OS-X) interface when you have to add another layer of complication on top just to use it?
The iPhone interface would scale easily and is similarly familiar but with no need for the extra layer. The advantage of using one OS over another is usually to leverage software, it would make sense to leverage the iPhone software more than the desktop software.
The tablet would also have to use the infra-red based hand and finger detection patent Apple published recently, to even make it *possible* to put OS-X desktop on a tablet, and there doesn't seem to be a good reason for it IMO.