Binary package hint: xorg
There is one mayor inconsistency with the fast-user-switching UI.
The unlock screen dialog now already shows a "switch user" button. What it does not yet show, but needs to show to avoid confusion is a list of already running user sessions, which are available to switch to.
Without that, a user might try to login twice. This might happen if user A locks his screen and then leaves the machine. User B logs in and leaves as well. User A returns and thinks he has been logged out/killed via Ctrl+Alt+Backspace and tries to login again. Moreover the way it is now, he would have to wild guess which VT his session is on and he would have to know about the appropriate keyboard shortcut, which is very unlikely with newbies.
By the way, I very much like the fact, that in Ubuntu it is actually taken serious if you say you can't expect a newbie to know - unlike in Debian. I know, and most people that would report a bug or wish here, but our mothers don't.
But when user A comes back and switches user he wil go to his open session by default, GDM shos the "Already logged in" subtitle.