The first commercial pc emulation software was mac-based and is still being revised and released:
http://www.connectix.com/products/vpc4m.html
Currently the OSX version is a "preview" but aside from a little networking issue (can't browse MS servers from within Virtual PC), it's fine.
For the daring, if you have X-Windows installed, you can run Bochs, though it's considerably slower than VPC and very unstable.
Connectix plans a true release of VPC4 for OSX soon, but if you want to use the preview, you have to buy VPC4 and then apply for the preview.