[1]The GDM login screen in Ubuntu Karmic displays the user list by default, which is considered a security risk by many people.
If you wish to hide this user list, and manually enter your username, run the following command in a terminal:
sudo -u gdm gconftool-2 --set --type boolean /apps/gdm/simple-greeter/disable_user_list true
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http://www.webupd8.org/2009/12/remove-user-list-from-ubuntu-karmic-gdm.html [2]