edit: I might add that any system that wants to be compatible with Windows 7 or XP needs to have the ability to disable Secure Boot. That fact will probably have larger effect on manufacturers than Linux users.
But MS could update Windows 7 & XP to be compatible, right?
Even if to begin with it is not a problem and windows 8 does not require secure boot to be enabled for compatibility with older computers it is still possible that windows 9 or even a later service pack will change that once they are happy that there is sufficiently few older computers.
This could create a market for motherboard modification that would allow pirated versions of windows to run.
The whole reason this has come about is due to the vulnerability of people faking BIOS IDs and manufacturers on startup, allowing auto-authentication of pirate copies of Windows Vista and 7. In order for this auto-authentication to work in Windows 8 on a shipped bundle, secure boot will need to be activated. After the initial secure boot and activation, hopefully it can be turned off and allow more open access on OEM machines. (If it doesn't, then we have a problem.)