One way to make a diverse set of computers feel more like a single environment would be always-on screen sharing. Say I have my home computer on while I'm at work, and they are both connected to a VPN service of some kind. Normally, while at work, I would be using the desktop there, but if I want to (in this scenario) I could activate a feature that zooms out and shows me both desktops at once. I can click on my home screen, which zooms in again to work on that remote machine, then zoom out and click back to my work machine. Add another machine to the mix, like my netbook, and the zoom-out adjusts to show that screen too. The borders between machines start to blur, and we would start to get a real-world idea of how people would use their computers if they had one environment spread across all their machines.
Mokalus of Borg
PS - Security, of course, would be a big concern.
PPS - And this assumes "one user, many machines" rather than the common "one machine, many users".