Honestly I was shocked when I read John's recent post about "Are VHDs compatible between Hyper-V and Virtual Server and Virtual PC?" Hestarts off with some "No, not supported" cases and I almost immediately started felling nauseous. Thankfully I read the full post. Towards the end there is relief:
Now let’s look at some “yes” scenarios. These are the ones where the virtual machine is built under Virtual PC or Virtual Server initially. If you take a VHD image and create a new VM in Hyper-V usng that VHD and simply boot it, yes, it will be fine. If you do not install the Integration Components and subsequently take the VHD back to Virtual PC or Virtual Server, yes, it will also boot fine.
Guess you also want to know what will not work. So go and check John's post.