Last week Dan Knight, of Low End Mac, speculated that an Apple tablet device with a 10" touchscreen would have an Intel CPU running Mac OS X, with iPhone OS also supported via what he terms virtualization. (Note that there has been an iPhone emulator built into Xcode ever since the iPhone SDK was released, so what he suggests is entirely plausible.)
If there are to be two different Apple tablet devices, it might be that one (most likely the smaller) will contain an ARM CPU running iPhone OS and the other an Intel CPU running Mac OS X, or something closer to it (including both AppKit and UIKit), perhaps with an emulation environment for running iPhone OS apps.
Incorporating this into what I've already predicted, we have two distinct devices (not two versions of the same device):
1) one with a 7-inch touchscreen, running iPhone OS on an ARM CPU, including a software-defined radio unit for long-range data connectivity, and enough battery capacity to support at least three hours operation with that power-hungry datalink continuously active, and
2) one with a 10-inch touchscreen, running Mac OS X on an Intel CPU, with Wi-Fi and Bluetooth, but no long-range radio circuitry.
These would be aimed at very different use patterns. The former would be a mobile platform and wireless network node, perhaps also providing connectivity to a laptop computer, whereas the latter would be for use in the home and other environments where it was never far away from a power outlet or a Wi-Fi signal.
It would be the larger unit, with the Intel CPU, that might run both Mac and iPhone applications. This device would be quite at home displaying a movie streamed from a Mac or Apple TV, or equally so accompanying a doctor through hospital rounds, but it would be the smaller one that went into your pack when you were setting off for a camping trip.
I'd expect the smaller device to default to portrait orientation, and the larger one to landscape.
I'd also expect the larger device to be released first, since the 4G networks that would make the smaller one so interesting are only now being built out, or are still in planning, and since the larger one is a better fit to a tie-in with music industry efforts to reinvigorate the multitrack album format, as well as a better tie-in with the release of Snow Leopard.
So, what we might see is a larger, Mac OS X device released as soon as September, and a smaller, iPhone OS device sometime next year.
Apple has an abundance of experience with processor emulation and code translation (MC68000 on PPC and then PPC and ARM/Thumb on Intel), with virtual environments (Classic, Rosetta, and an in-house JVM on Mac OS X and the iPhone simulator in Xcode), and with hardware abstraction (the Acceleration Framework, OpenCL, and Grand Central Dispatch), as well as with touch interfaces (iPhone and iPod touch), so there's no reason to think either device described here would present an insurmountable challenge to them.