Hidden within the usual litany of claims in a patent awarded this January—titled "Display Housing for Computing Device", which covers their current portable displays—Apple describes a tablet display, with the display housing the computer device.
The patent describes several forms for these displays—most of them already in use in current Apple laptops—, describing displays attached to portable computers. However, one of the claims describes the display as a "housing for a computer device", which would include the computer electronics within the display housing, sandwiched between front and back shells.
 As a housing for a computer device, one embodiment of the invention includes: a front shell; a back shell coupled to said front shell to produce said housing, electrical components for the computer device being internal to said housing; and a foam stiffener provided internal to said housing to substantially fill unused space internal to said housing, thereby providing stiffness to said housing.
According to our patent expert R. Polk Wagner, Professor of patents law at University of Pennsylvania Law School:
What might be fair to say is that Apple's patent is significant in that it shows something about the company's thinking: that they are thinking about "portable computing devices" where the display (and/or keyboard) are larger than the volume required to house the components inside - thus allowing or even requiring a stiffener. Maybe this means Apple is thinking about tablets (i.e., "big iPhones") or maybe they're just thinking about further miniaturisation of components / lighter weight in existing form factors. Hard to know.
So there you have it. To me, the patent claim quoted above leans clearly to a potential tablet device. In any case, remember that patents don't have to materialise into products, but it's always comforting to see Apple thinking about creating such a device. Or at least, contemplating the possibility seriously enough to cover all bets in a patent. [USPTO via Electronic Pulp]