Are Windows 8 ARM tablets actually computers?
Windows 8 for ARM (unsurprisingly) only runs on ARM machines. This also means that (almost) none of today's computers will be able to run this version of Windows. They are stuck with the normal, full-featured Windows 8.
Windows 8 for ARM requires manufacturers to enforce Secure Boot, Microsoft's newest technology for ensuring a trusted boot sequence.
On non-ARM systems, it is required to implement the ability to disable Secure Boot via firmware setup. A physically present user must be allowed to disable Secure Boot via firmware setup without possession of Pkpriv. Programmatic disabling of Secure Boot either during Boot Services or after exiting EFI Boot Services MUST NOT be possible. Disabling Secure MUST NOT be possible on ARM systems.
(Source, emphasis mine)
Microsoft requires manufacturers desiring a certification to prevent the system from disabling secure boot.
I haven't been able to find recent information about this, but it looks like it's at least very difficult to get other OSes for ARM to run on these tablet devices, and it might even be impossible.
We've had Linux on Macs for years, even back when it was PowerPC. Now, Linux, Windows and OS X can run side-by-side on Macs. There's never been any issue with running Windows or Linux (or Solaris, or BSD, ...) on regular computer hardware.
From that perspective, ARM tablets look a lot like iPads with their missing versatility and vendor-lock-in.
Is Windows 8 for ARM a computer operating system?
When Apple switched from PowerPC to x86 processors, they developed an x86 version of Mac OS X Tiger and provided an emulation platform - Rosetta - that allowed execution of PowerPC software on new Intel Macs. There was no doubt that the new x86 variant was as powerful as the PowerPC one.
Windows 8 for ARM (actually Windows RT) on the other hand is severely limited and cannot be considered an alternative to existing Windows systems. It only supports execution of applications specifically designed for Windows Runtime, Microsoft's 2011 programming model for Metro applications.
Approximately 0% of today's Windows software is compatible with this new programming model. None of your software will work on Windows for ARM. It's not just a reduced feature set of what was there before: They developed something from scratch for the tablet use case. It's simply not as obvious as with iOS, because Microsoft reused it for their desktop product as well.
This looks a lot like Apple's approach to iOS: Cherry-pick a few nice things (like .NET), throw away decades of crud, and start with a blank slate. Of course, Metro is used on desktop Windows 8 as well, but integrating the iOS simulator well with OS X's GUI wouldn't make iOS any more of a desktop OS either!
Recap
- Windows 8 ARM tablets will not run other operating systems.
- Windows 8 for ARM does not support other hardware (due to secure boot requirement).
- Windows 8 for ARM supports none of today's Windows software
- Software has to specifically target Windows 8's new WinRT/Metro model to run on Windows 8 for ARM.
I completely agree with @slhck:
If the tablet runs a desktop OS, it's on-topic for Super User.
It's just that Windows 8 for ARM isn't a desktop OS. And it's not just the form-factor.
Should we care?
It's always been the goal in Stack Exchange to make sites that target a specific user group. This results in some topic overlap (Apple.SE and SU come to mind, as well as SU/Unix&Linux/Ubuntu.SE), and some topics not handled at all (like with the recent BlackBerry question on this Meta). This has been discussed before and found to be useful.
As a user, I should be able to participate on as few or as many SE sites as I want, and not be sent around from site to site, just because some other site exists for a particular topic. To that end, sites require easy to understand policies that make sense.
Just because there's Ubuntu.SE doesn't mean Ubuntu is off-topic on SU. And that there's no BlackBerry SE site doesn't mean we accept questions about it on SU either.
The only possible reason I can imagine for allowing Windows RT (Windows 8 for ARM) questions on SU is the fact that there's no other site for it on the network.
Beyond the simple issue that this has never bothered us before, because a site's policy needs to make sense without considering other sites on the network, it's short-sighted: Allowing Windows 8 for ARM will reignite the discussion about Android and iOS (at least the tablet variants) in a big way. And I have no idea what a good answer would look like, other than "Yes, we now accept questions about all tablet computers and their software".
I can understand your enthusiasm about Windows for ARM, but arbitrarily changing how we view tablet computers on SU is not the answer.