This method works for Ubuntu 12.10
With latest 12.04 release there is no option to login as other user on login screen. Normally this is not required as login for all users appears on login screen.
This is required only when you try to login as root user.
To enable other user login, run following command.
sudo echo "greeter-show-manual-login=true" >> /etc/lightdm/lightdm.conf
We do not encourage readers to login as root, login with non-root user will keep your linux secure.
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