Steven Sinofsky, head of Windows, wrote a blog post titled Running Windows 8 Developer Preview in a virtual environment where he goes into the subject in detail. It's a good read. Halfway down he says:
"... our baseline assessment is as follows:
Functional:
- Hyper-V in Windows 8 Developer Preview
- Hyper-V in Windows Server 2008 R2
- VMware Workstation 8.0 for Windows
- VirtualBox 4.1.2 for Windows
Non-functional:
- Microsoft Virtual PC (all versions)
- Microsoft Virtual Server (all versions)
- Windows 7 XP Mode
- VMWare Workstation 7.x or older"
So no, Microsoft Virtual PC does not official support Windows 8. If an unofficial hack comes out and gets it running, it will probably be a sub-par experience, but a cool trick none-the-less.
Update: It does seem odd that they wouldn't support their own product until you realize that they are getting rid of Virtual PC in favor of Hyper-V which will be included with the Windows 8 client. So it's actually an upgrade.