This blog post explains how to disable the user list at the GDM login screen. The solution presented here was tested on Ubuntu Karmic Koala, but it might work on other Ubuntu versions as well (Hardy, Interpid, Jaunty, Karmic and Lucid).
To disable the user list, run this command in a terminal window (without the $ signs):
$ sudo -u gdm gconftool-2 --type=bool --set /apps/gdm/simple-greeter/disable_user_list true
Logout and login again. You have to type your user name from now on.