In newer versions of lightdm there is a guest-session option in the config. Otherwise you should set your default session to the one you prefer.
** Changed in: lightdm (Ubuntu) Status: New => Fix Released -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to lightdm in Ubuntu. https://bugs.launchpad.net/bugs/985724 Title: LightDM doesn't use GNOME Shell as guest session after uninstall of Unity Status in lightdm package in Ubuntu: Fix Released Bug description: LightDM doesn't automatically use GNOME Shell as guest session after uninstall of Unity. There is not even a graphical user interface for setting the default session. When a user removed Unity, but did not change any setting manually and tries to start a guest session, the user will see just a plain background image with no interface at all. Only Ctrl+Alt+Delete allows to logout. As a workaround you have to manually edit "/etc/lightdm/lightdm.conf" and change "user-session=ubuntu" to "user-session=gnome-shell". ProblemType: Bug DistroRelease: Ubuntu 12.04 Package: lightdm 1.2.0-0ubuntu2 ProcVersionSignature: Ubuntu 3.2.0-23.36-generic 3.2.14 Uname: Linux 3.2.0-23-generic x86_64 NonfreeKernelModules: nvidia ApportVersion: 2.0.1-0ubuntu5 Architecture: amd64 Date: Thu Apr 19 17:49:21 2012 EcryptfsInUse: Yes InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111011) SourcePackage: lightdm UpgradeStatus: Upgraded to precise on 2012-04-17 (1 days ago) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/985724/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp