Re: "ability to run headless virtual machines"
I was just going to say, I was doing this with VirtualBox a couple of years ago - and it works very nicely indeed :-)
You can manage your headless machines through a very feature rich web interface that is almost as good as the desktop manager. Or do some extremely advanced things via the commandline (which can manipulate running VMs as easily as offline ones)
It really is a rather incredible bit of kit :-)