I had a user ask for Windows 8, so that they could assist customers through our processes in the new Windows 8 environment. We are not ready to deploy Windows 8 as an organization, but for testing I figured it would be ok and a great way to use the Windows Virtual PC role on their Windows 7 system!
I figured wrong. After initially creating a blank Virtual PC on my Windows 7 system - to build the Windows 8 machine, I popped in the Windows 8 32bit ISO file in the virtual CD/DVD drive, turned on the Virtual PC, and I was greeted by the following blue screen frowny face.
Long story short, you can use Windows 8 on other Virtualization software - such as Hyper-V, VirtualBox, VMware, etc. - just not Windows 7's Virtual PC.
I ended up creating a VirtualBox Virtual Machine for the user. When I did this a few weeks ago, there was still some integration issues with VirtualBox with the sharing of files, attaching USB devices, and clipboard, but it works well enough for her to test.
Your mileage may vary. Good luck!