I was playing with the themes in Ubuntu, installing and uninstalling GTK engines, and for some reason, the theme that Ubuntu uses to show the login screen got changed. After some research, I discovered how to restore that theme. This problem could also appear to you if you installed xubuntu-desktop or kde-desktop, because those packages install their own login window themes and substitute the Gnome default login theme with them.
All right, to solve this problem, open a terminal an enter these 3 commands:
sudo -u gdm gconftool-2 -t string -s /apps/metacity/general/theme HumanLogin
sudo -u gdm gconftool-2 -t string -s /desktop/gnome/interface/gtk_theme HumanLogin
sudo -u gdm gconftool-2 -t string -s /desktop/gnome/interface/icon_theme HumanLoginIcons
These commands substitute whatever theme is being used right now by the GDM login screen with the default theme that GTK, Metacity and the icons used.
Now logout and see if it worked.