Multi user multitouch operating system and monitor

Now that multitouch technology is available in common operating systems (MAC OS, Windows 7, Ubuntu 10.10 beta), it would be great to allow two users simultaneously to use the same computer (same screen, different controls, same or different applications). An example, using one computer with one big screen, I would read my emails in a browser program and simultaneously my friend would write a letter in a word processor program. We would need in total 2 keyboards and 2 mouses, or we would use simultaneously a touch screen. Such a touch screen or system would have to recognise the different users. May be by measuring the body fat percentage, by observing user gestures with a camera, or by voice recognition.