I don't think it is accurate to say that "any ARM device that ships with Windows 8 will never run another operating system unless...". First, it should be changed to "any ARM device with the 'Designed for Windows 8' logo will never run another operating system unless..." since it isn't the presence of Windows 8 that determines the status of secure boot. Second, the "unless..." part is pretty important and should be emphasized. Just as Microsoft will be able to sign Windows, other organizations will be able to sign their own operating systems. And hacking/jailbreaking will always happen. So it is more accurate to say "any ARM device with the 'Designed for Windows 8' logo will have to be jailbroken before it can run any unsigned operating system".
In any case, it seems that Microsoft's rules here are the most open of any tablet manufacturer. Can you install your own copy of Linux on an iPad? No. Can you install your own copy of Linux on an Android device? Only if the manufacturer was kind enough to leave your bootloader unlocked. Can you install your own copy of Linux on a "Designed for Windows 8" device? Yes, as long as you get it signed first.
Will you be able to buy Windows 8 for ARM on a tablet that doesn't have the "Designed for Windows 8" logo certification? If so, then I imagine there will be a lot of vendors willing to forego the logo certification and advertise the tablet as "Linux-compatible". Of course, in that case, I would hope they would advertise that they passed all other "Designed for Windows 8" logo requirements. On the other hand, if Windows 8 for ARM is restricted to OEMs selling properly-certified tablets, OEMs will probably be less likely to sell Linux-compatible variants.
On what terms will 3rd parties be able to sign their operating systems? There will definitely be ways to do it, but it won't be free. Getting the OS signed will probably have a fee, and nobody will want to have an OS signed in their name unless the OS is a "closed system". In other words, I would be very nervous about getting a standard GRUB binary signed in my name, because anybody could then take that GRUB binary and use it as part of a rootkit, and it is not beyond the realm of possibility that I might be held liable for damages done by that rootkit. Instead, I would (at the very least) want to modify GRUB so that it shows a splash screen saying "Warning: this version of GRUB can load unsigned operating systems!". Or if the lawyers have their way, I would probably make my GRUB only load signed kernels so that I can pass the blame to whoever signed the kernel. Anybody signing a kernel will probably want to have the same attitude towards unsigned kernel drivers (and probably even unsigned user-mode drivers, since they usually have special capabilities and extra potential for causing mayhem).
How interested will people be in jailbreaking these devices? So far, the best tablets (the ones on which people want to install their custom builds of Linux) are NOT the "Designed for Windows 8" tablets. Will this change in the future? Microsoft's policy only matters if "Designed for Windows 8" tablets become the best tablets on the market. So far that is not the case. If this does wind up being our future, it is at least several years away. What will the tablet market look like then? Will "Designed for Windows 8" tablets really be the only game in town for your next Linux tablet, or will there be other options?
Will you really want to put your own build of Linux on these tablets? I mean, I love tinkering with my desktop and laptop computers as much as the next guy, but phones and tablets are not general-purpose computers -- they're appliances.
Will the major distro providers (Ubuntu, Debian, Red Hat, FreeBSD) step up to the plate and release signed versions of their distros? Will businesses see enough need for custom operating systems to build and sign a Linux image for internal use?
How does code signing play with various open-source licenses? Is it ok to sign a GPLv2 program? What about GPLv3?