In an earlier article, I've written about Microsoft's requirements for computers if their makers want them to get Microsoft's stamp of approval for Windows 8. Those requirements open the possibility that such computers will be rendered incapable of booting any operating system other than Windows.
Now we find that for ARM-based systems, it's going to be more than a possibility. For x86 based computers, Microsoft apparently is willing to approve of systems using a "custom" secure boot mode that will allow its owner to boot other operating systems. For ARM-based systems, though, Microsoft explicitly states that they will not approve any system with "custom" secure boot mode; they must be set so that they can never boot any operating system other than Windows.
ARM, at least currently, is the predominant CPU family for mobile devices. Intel has yet to put out an x86-flavored CPU anywhere near as efficient in terms of power consumption versus processing power. These days you'll find a lot of people claiming the desktop is on the way out--and Microsoft is looking to lock out competition so it can dominate mobile devices the way it has the desktop.
For details, see the Software Freedom Law Center's article, and Glyn Moody's article in Computerworld UK.
UPDATE: Even on the x86, the constraints will make life quite difficult. Check out "Why UEFI Secure Boot is Difficult for Linux".