Secure boot is a feature of the Unified Extensible Firmware Interface or UEFI, the replacement for the BIOS on the motherboard. The implementation of secure boot by Microsoft for Windows 8, using cryptographic keys for authenticating the kernel that is being loaded, means that any other operating system looking to load itself on a secure boot-enabled Windows 8 system has to also be able to authenticate itself.
During my initial look at openSUSE 12.3 I concluded that while it had support for secure boot and could be installed on a Windows 8 box which had an additional drive, there was no way of accessing the installed system.
This is not correct. One can access the system, provided all previous boot entries from previous installs of other operating systems - in my case, Fedora and Ubuntu - are removed from the list of bootable systems. This can be done easily by using an application called EasyBCD which runs on Windows 8.
After the installation is done, and one reboots, the machine will come up a menu that presents openSUSE as the default and offers Windows 8 as an option. But Windows 8 will not boot from this menu.
One needs to go into the UEFI and change Windows 8 to the default choice for booting. Then only can one boot Windows 8.
After this is done, to access the openSUSE system, one needs to boot into Windows 8. Then click on restart while holding down the shift key. The screen that comes up offers an option to select other systems for booting and once that is selected, on the next screen an option for openSUSE presents itself.
The documentation is an excellent read and one more indication of the professionalism of the team that develops and markets openSUSE; reading it would be useful for anyone who wants to understand the new constraints that Microsoft has placed on running GNU/Linux.