I recently downloaded the September CTP VPC image of Orcas (aka Visual Studio vNext) only to find that the image I downloaded refused to mount in Virtual PC.  Thankfully I just stumbled across Jon Galloway's post about just this issue here.  In a nutshell, there are two VPC images I needed to download; I only snagged the base image (which contains the operating system) and missed the image containing the actual Orcas bits.  Note to self, always read the (full) instructions.  I'm a happy camper now and will be posting initial thoughts relatively soon.