Craig, when you look in Finder, does your boot camp volume appear in the "Devices" list in the left-hand sidebar? I was able to reproduce the error message "If another virtual machine is using the disk..." by selecting the Boot Camp device in Finder and doing a right-click "Eject" on it (thus dismounting the volume). Restarting my Mac did put the BOOTCAMP volume back into Finder, and restarting Fusion after that let me create a Boot Camp VM from it. Is your boot camp volume visible from OS X at all?
The engineer who knows most about this is out today, but we'll follow up in the morning.
