https://wiki.ubuntu.com/UnifiedLoginUnlock
кратко:
There will be a single screen used for logging in, user switching, and screen unlocking. This will be the gdm login screen, possibly as modified by FaceBrowserLogin.
ну это ладно. унифицация - это хорошо.
Logging in will automatically switch back to (and unlock) an existing session if there is one, or start a new one if not.
You will never be offered a new session when you already have one unless you manually start an X server from the command line.
а вот с этого места мне нравится перестало.
Each user session will run in a separate "proxy" X server; likewise, gdm will run in a similar proxy. These proxy servers will all connect to one single real X server.
When the real X server provides GL and if Xgl works properly, the proxy server will be Xgl. When it does not, it will be Xnest or perhaps Xephyr.
а вот это уже, имхо, полный алярм