I've got 11.10 on a desktop machine with two users. Both users have "Password" set to "Not asked on login" in Users Settings. At startup, either user can log in without a password. But once both users are logged in, it takes a password to switch between users.
In previous Ubuntus, you could override this by setting /desktop/gnome/lockdown/disable_lock_screen to True in gconf-editor. That is ignored in the Gnome 3 / Unity era.
Does anyone know a way to disable the password prompt when switching between users in Oneiric?