I am not completely sure about this statement. Surely there is a great
part of the setup that is too low level to be considered here. But it
would be nice to come out with a specification of how the user should
perceive the possibility of using multi-seat in the graphics shell
(that is unity). There is some similarities with the support for
multiple monitors in Unity, I believe.
Some questions that come to my mind:
1) How should Unity behave if it sense two (or more) graphics cards
with monitor attached? Should it treat them as a multi-monitor or a
multi-seat system?
2) How should a wizard do configure multi-seat should look and feel?
3) How to transfer the permissions to mount new USB or other
hot-plugged devices?
Answers to those questions seem, IMHO, associated to this list.
Anyhow, any discussion should be preceded by the discussion if it is
desirable to treat multi-seat as a major feature in the Ubuntu system
or not.