It turns out that it's possible to make a single file that is both a bootable ISO image that can be burned to a CD, and a bootable hard-drive image that can be written to a mass storage device (like a USB key), to create a storage device that is bootable on x86 machines with a BIOS, x86 machines with EFI, or PowerMac machines.

It's possible, but it's not pretty: http://mjg59.dreamwidth.org/4957.html