Fusion 5 supports Windows 8 Boot Camp.
The problem I think you saw, a failure at the point of "VMware Fusion is preparing to run as a virtual machine" can be caused by the state of the Windows registry. While I don't have a fix for that right now (we have a bug filed, and are working on it), I do have a workaround.
The problem occurs after the Boot Camp VM has been set up, but before it's powered on for the first time as a virtual machine. Fusion is attempting to insert registry keys and enable drivers for the boot device, and in certain situations it's unable to do so. As a workaround, you can add those keys and driver enablings (!) manually, under Boot Camp itself. They don't interfere with using Boot Camp natively.
If the problem occurs, you should
1) Find the file /Applications/VMware Fusion.app/Contents/Resources/naos-1.0.vmwarevm/reconfig.iso and open it under OS X, which will mount the ISO.
2) Copy the file "bootcamp.reg" on the ISO and copy it to some folder on your Mac, or to a USB drive. We'll want this file from Windows.
3) Restart your Mac under native Boot Camp.
4) From Windows, find the file bootcamp.reg where you saved it, and double-click to add those settings to the Registry.
5) Restart your Mac under OS X, start Fusion, and recreate the Boot Camp virtual machine.
6) It's likely that it will fail again at the "preparing your virtual machine" stage, but that's no longer critical since the necessary settings have been added to the registry already. What we do need to do is prevent Fusion from deleting the VM when you acknowledge the failure. We'll do it by renaming the VM.
7) While the error alert panel is showing, use Terminal or Finder to rename the VM. You'll find the Boot Camp VM in your home folder, under ~/Library/Application Support/VMware Fusion/Virtual Machines/Boot Camp. Note that the ~/Library folder is hidden by default under OS X Lion and Mountain Lion, so the simplest way to get to this folder might be from Terminal, not Finder. If you'd like to do the renaming via Finder, open a Terminal window and enter the command "open ~/Library/Application Support/VMware Fusion/Virtual Machines/Boot Camp" and Finder will open the ordinarily hidden folder for you. Rename the Boot Camp VM bundle, which will prevent Fusion from deleting it as a failure, then acknowledge the alert panel. You can either rename the Boot Camp VM back to "Boot Camp", or simply drag it into the Fusion VM Library under the name you gave it.
