The login screen that is reached via Switch User, does not allow you to log back into a active session. The only way to get back to the active session, is via manual VT switching (at which point you will find out the session was never locked either, however that is probably a different bug. http://bugzilla.gnome.org/show_bug.cgi?id=:684459)
I am filing this against gnome-shell as it handles the switch user screen, however it would also involve gdm and polkit to some level.
Steps to Reproduce
1. Click 'Switch User' from User Menu
2. You will get to the login screen
3. Try to log back in with the same user, after entering password it silently fails and goes back to 2.
ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: gnome-shell 3.5.92-0ubuntu1
ProcVersionSignature: Ubuntu 3.5.0-15.22-generic 3.5.4
Uname: Linux 3.5.0-15-generic x86_64
ApportVersion: 2.5.2-0ubuntu4
Architecture: amd64
Date: Fri Sep 21 06:54:07 2012
GsettingsChanges: b'org.gnome.desktop.interface' b'icon-theme' b"'Humanity-Dark'"
InstallationMedia: Ubuntu GNOME Remix 12.10 "Quantal Quetzal" - Alpha amd64(20120902)
SourcePackage: gnome-shell
UpgradeStatus: No upgrade log present (probably fresh install)
Attached upstream bug with fix, it is awaiting freeze break approval, but should make it into 3.6.0