Had this problem too, found a solution.
1) Download the 64bit Windows 8 Enterprise Evaluation. (http://msdn.microsoft.com/en-us/evalcenter/jj554510.aspx)
2) Go through the Boot Camp process, selecting the downloaded .iso as your image.
3) As soon as your computer restarts, hold the option key. Boot back into OSX.
4) Go through the boot camp process again, BUT only ticking the first two options (in other words, don't touch the partition) selecting your original Windows 8 copy as your image.
5) Reboot while pressing the option key, select the (in my case) USB.
6) Proceed as normal - worked for me.
I was able to download via Microsoft. The downloaded file was in a strange format but after changing it to .iso, it showed setup.exe which is what I recognize as Windows installer image.
The problem: I can't use the downloaded .iso image. Boot Camp Assistant requires me to insert a disc or a flash drive version of the iso.
The rest of the installation process I'm used to, I just can't get the right setup file to start the process.