Part of the issue with Windows 8 on ARM is how there is a lack of standards for that architecture. OEMs must adopt OS versions, Windows 8 included, to work on a particular chipset and system package. That prohibits the ability of Microsoft to make Windows 8 available directly to end-users for self-installs.
A bigger issue is one that I don't see a way for either OEMs nor Microsoft to circumvent, and that's the OS install process for ARM devices. Just like we see on phones, loading a new OS version on ARM tablets/laptops will be a firmware update. This means the entire system is wiped out prior to replacing it with the new OS update. As many device owners can attest, anything interfering with this wipe/update process can leave a device no better than an expensive brick.
I can't believe that Microsoft will be willing to take the chance letting users install Windows 8 on ARM devices. There is too much exposure to expensive install problems, and it makes sense for the company to handle it like it does on Windows Phones. OEMs will bless a ROM image for a given ARM device, and then push it out to the device in an official OTA update. Even worse, Microsoft may choose to only make Windows 8 available pre-installed on ARM devices. Otherwise it just isn't going to happen, I don't believe.