Hi!
I got 8.1 installed on my MBP!!! Very easy, almost like the original Boot Camp how-to, no tricks and heavy lifting needed as described by several posters on this thread (at least for me!).
A few days ago I bought my first ever apple product, a MBP (Quad-core i7 2.3GHz, 16GB, 512GB, Iris Pro Graphics, GeForce GT 750M 2GB). I had heard that BootCamp should be simple to use and help me get Windows 8.1 running (since I am not really interested in OSX). My main usage of the machine is for programming and this one is bought only for it's hardware spec vs weight (very light, nice when travelling a lot).
Being a complete retard with OSX, I had picked up somewhere that the Boot Camp softeware should be easy to use and help me get Windows on the laptop. This is probably true for older versions of Windows. I started out with the Windows 8.1 ISO image, following the instructions given in the boot camp how-to, and I ended up in the same state as everyone else in here. When I saw that the windows installation could not write to the boot partion, and I could not get anywhere, I simply restarted the machine. It booted up into EFI (I did not know that yesterday) and the screen was completely white. I was horror-struck, as I did not see any information and I had no idea what to do (besides to google on my other laptop). It turns out there is an option key that must be pressed down when booting, so you can choose boot options, how intuitive? Why not simply state that on a pre-boot screen, like press 'del' to enter bios?
Anyway, I then found this thread and read through it, and I got a little worried that I would have to wait to get this working, my backup alternative became to install parallels and run Windows from there. Kudos to the guy who mentioned that workaround.
But before I completely gave up on Boot Camp, I decided to try with a Windows 7 image:
1. Updated everything in the OS (maybe there was a Boot Camp update, I don't remember)
2. Downloaded the Windows 7 ISO image (from MSDN).
3. Inserted the usb memory stick
4. Started Boot Camp
5. Followed the standard instructions given by the Boot Camp wizard, hooking off all three alternatives
6. Boot Camp downloaded and updated all windows drivers etc.
7. Boot Camp formatted and installed the usb memory stick.
8. Boot Camp created the Windows partion, I resized it to 80% of harddrive capacity.
9. Boot Camp booted into EFI/memory stick and the installation started.
10. I simply formatted the BootCamp partition (no deletion of BootCamp partition needed).
11. Windows 7 installed itself and wrote whatever needed to the boot partition.
-- Wow, at least Windows 7 in there.
12. On the USB stick, all the Apple drivers for windows are located, under the folder BootCamp. I ran the setup.exe and suddenly all the drivers on the machine started to work and I got wifi connection.
13. Downloaded Windows 8.1 ISO image from MSDN.
14. Downloaded and installed Virtual Clone Drive (VCD).
15. Used VCD to load and run Windows 8.1 image
16. Windows 8.1 installtion software updated the Windows 7 OS successfully to 8.1.
17. Running the setup of the Boot Camp drivers again to get better screen resolution, wifi, all the drivers installed.
That was it! A little effort going through the Windows 7 route. I guess that you could probably get this to work even with a Windows 7 pre/beta -release as well and then upgrade to 8.1 (then you don't need to purchase Win7 just to get this to work).
Hope this helps anyone out there,
Steinar.