Just upgraded to Fedora 18 yesterday and now I notice that, I can't use switch user entry. When I click on switch user, my screen lock up (I get that screen where I have to hit enter/Esc or mouse wheel up, eg. lock screen), but there are no other users listed. Only option is to log in as me again (enter my password).
After latest updates (including gnome-shell 3.6.2-3.fc18.x86_64 ), this problem seems to be fixed. So I'm closing it.