Unfortunately, Gnome 2.28, which comes with Ubuntu 9.10, uses the dropdown list by default, and what's worse, doesn't give you any easy and intuitive way of changing it. That's just sad.
In order to modify this, you'll have to open a terminal window and execute:
sudo -u gdm gconftool-2 --set --type boolean /apps/gdm/simple-greeter/disable_user_list true
Idea from Ubuntu Tutorials: Remove the User List from Ubuntu 9.10 GDM.