A rather flexible solution to multiseat is remote virtualization (i.e. running the virtual machines at some remote server rather than locally on a desktop machine). Of course, you will need to access the virtual machines remotely, e.g. over RDP for Windows guests (or for accessing VirtualBox console regardless of the guest OS) or VNC (KVM console, Unix guests). I'm very excited about SPICE and will be trying to move to SPICE-enabled virtualized desktop at home to see how it fares.
Then, of course, one needs a computer to access the remote virtual machine. I think RaspberryPi should be able to handle an RDP/VNC/SPICE client while being cheaper at the same time! I'd like to buy one (or three) of those once the 350000 preordered devices have been shipped
Such scenario is not strictly speaking multiseat but achieves pretty much the same effect, if you can find cheap client machines like RaspberryPi. And you get 'hot desk' (is this a real idiom?) along the way.
I think virtualization can be used even with a traditional multi-GPU mutliseat configuration. It's just that the only application that will be run on each seat is an RDP/VNC/SPICE client which connects to the virtual machine for the particular user possibly running on the same physical machine. That's pretty nice if you want to implement multiseat for, e.g. Windows or some other OS that cannot do multiseat.