In my view, the tablet and the PC are different. You can do things with a tablet if you aren't encumbered by the legacy of a PC--if you view it as different. If you say this is another PC, all of a sudden you're pulling all of the lead weight of the PC market and you wind up with something not as good.
Are tablets PCs or are they something different? Microsoft thinks they are PCs, hence it is putting roughly the same version of Windows 8 into desktop and tablet computers. Apple thinks they are different. Presumably, in spite of some commonality of desktop apps in Mountain Lion, there won't be some grand convergence of Macs and iPads coming.
I confess, I could argue either side sincerely. I've already argued there are fundamental differences between touch apps and conventional desktop apps and it's not clear how to bridge the gap. And yet it's impossible to deny that users are buying these devices mostly to do things they might otherwise have done with computers. It's not like with phones, where there is a distinct and important function they have which PCs lack.
Some research firms like Canalys are even counting tablet market share as part of a total computer market share. If they aren't the same market why do this? Maybe because it makes their numbers look more spectacular and it gives them fewer categories to count, but maybe they just agree with Microsoft. And if they are right, then tablets, especially iPads, are a huge factor in the PC market, bigger than the Mac ever was.
Let's assume, as I think is reasonable, that with tablets as with PCs the consumer and business markets will turn out to be distinct. Windows tablets should have a considerable advantage because so much existing expertise and infrastructure is designed to handle Windows systems.
The demand for it created a new market for manageability of mobile devices, generally known as MDM (Mobile Device Management), although there's a lot more to it than what have come to be traditional MDM features. These MDM systems operate outside of the existing management infrastructures in enterprises, in separate consoles. Enterprises can treat Windows tablets--on x86/x64 processors--as first class managed devices, not on some parallel management system.
That parallel management system won't go away, though; not only will it be necessary for the iOS and Android devices, but Windows on ARM systems won't be manageable. And in the best--and worst--traditions of BYOD, you just know that employees are going to come into the office with their slick new WoA tablets and demand to use them.
I envision a two-tiered world, where the critical apps remain largely on the managed devices. It will always be harder for the rogue devices, even the Apple ones, to keep up, but they might keep up just because users insist on it. It's only trouble for IT, and the whole point of BYOD is to disregard their interests anyway.
So in that sense, tablets are both PCs and big phones (without the telephony). It's possible that both types will survive, maybe even thrive in the market. Both Apple and Microsoft could be both right and wrong.