1. Boot machine while pressing F10
2. Find Secure Boot in the menu tree, ignore warnings
3. Disable Secure Boot feature
4. Enable legacy boot options
5. Enable specific legacy devices, such as USB devices
6. Save and reboot while holding down F9
All of this is from a Distrowatch piece by Jesse Smith. No doubt all of this won't work when I need it either, but somebody might Google it up and find it. Some things we must pay forward.
A follow-up: The currently favored solution is a workaround: a pre-bootloader signed by Microsoft (so it passes secure boot) that can then be used to load a normal Linux bootloader without further signature checking. One Linux developer, Matthew Garrett, has managed to get Microsoft to sign a pre-bootloader called Shim. You can download it today and use it to boot Linux on your Windows 8 machine. Shim should soon find its way into SUSE, Fedora, Ubuntu, and other major Linux distros.