It'd wholeheartedly be OSX. More or less whatever Linux can run on, some kind of Leopard will find it's way on it.
Just a word on these Slate things being sexeh. This is an offering Verison were showing off at CES, not sure on the 'real' manufacturer, probably something along the lines of HTC - but there are some funky dual ARM processing nonsense-umma-thingy and nvidia's new Tegra2 stuff-umma-something:
U know Apple's proposed offering would kinda look the same, but instead of being something u look at and think it's sexy - u'd also have an overwhelming need to pick it up and give it a squeeze. That's without even seeing what it is yet !
At the show they were demo'ing how these things run the Unreal 3 engine, 1080p and all kinda of crazy web flash 10.1 and whatever. A comment from a tech review guy who belongs to one of the partner links at the bottom of oc3d's pages, was along the lines of these things versus the new phones coming out - in so much that these things handled everything thrown at them with great speed, whereas the newer phones were still stuttering at places they used to.
I'd love a device like this in the front room. Full net access, touch screen hopefully, and access to (and control of) other pcs/mac/gadgets throughout the house. All wifi'd, bt'd. Ugh, noiceness.