virtualbox.org

1 Create new VM, Type "Microsoft Windows" Version "Windows 8.1 (64 bit)" and changing only "Enable EFI (special OSes only)" to enabled.
2 Create a 100GB VDI.
3 Point the IDE CD/DVD drive to the downloaded 64 bit ISO from Microsoft, Windows 8.1 Enterprise.
4 Start the VM.

I briefly see a "Press any key to boot from CD or DVD ..." and then I get an EFI shell. On FS0: there is only one file, README.TXT, but if I mounted this ISO in either OS X or Linux I see a bunch of other files and folders. So it's like the firmware isn't correctly reading the ISO. Any ideas?

From the vbox log:
00:00:00.458945 File system of '/Users/chris/Future NAS/ISOs/9600.17050.WINBLUE_REFRESH.140317-1640_X64FRE_ENTERPRISE_EVAL_EN-US-IR3_CENA_X64FREE_EN-US_DV9.ISO' (DVD) is hfs

What does "is hfs" mean? The ISO is definitely not any of the Apple HFS variant file systems. The host platform is Virtual Box 4.3.10 running on OS X 10.8.5.

https://www.virtualbox.org/manual/ch03.html#efi
"...Windows guests are currently unable to boot with the VirtualBox EFI implementation."

Filed bug.
https://www.virtualbox.org/ticket/12997

it's not a bug. it's not implemented yet. not sure why you would file a bug report for something that's already in the manual.