One option is to downgrade your version of GDM. But where's the fun in that? There's a better way..
So to get rid of it, pop open your terminal enter this handy-dandy command:
sudo gconftool-2 --direct --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory --type Boolean --set /apps/gdm/simple-greeter/disable_user_list True
Log out and then you should be greeted with the generic login screen you know and love.