Microsoft has been discovered to have changed its requirements for the upcoming ARM version of Windows 8. The change essentially will prohibit ARM devices, including PCs, from running operating systems other than Windows 8 after they ship to customers.
Specifically, Microsoft recently amended its requirements for ARM Windows 8 System Builders. Unlike Windows 8 for Intel-compatible (x86 & x64) machines, the ARM version of Windows 8 will not be sold to the public. To purchase an ARM version of Windows 8, you will have to purchase a device with it pre-loaded (similar to Windows CE devices today, such as Windows Phone). The new requirement calls for utilizing UEFI Secure Boot, a technology that forces manufacturers to instruct devices to boot code certified by the manufacturer for the device.
For consumers, this is similar to locking the bootloader on a smartphone. This is a common practice on mobile phones that secures the device, but blocks running modified or alternative operating systems. Many manufacturers now allow consumers to bypass (or "unlock") the bootloader lock on select devices (and at times, wireless providers).