Install XDM instead. ;)
Karmic ships with a new version of GDM (2.28) which is rewritten, and by default presents a list of usernames, in much the same way as XP does by default. Lots of people dislike this. It’s also currently lacking a graphical config tool (it is in beta…).
To change it, run this:
# gdm gconftool-2 --set --type boolean /apps/gdm/simple-greeter/disable_user_list true
This, I feel, is non-ideal since it just replaces the users list with a ‘log in’ Window and button which is completely superfluous – if I’m at the logon screen, I probably do want to log onto the PC, and the most logical thing for it to do is to be already asking for my username, ideally with that text box in focus. The previous login screen was pretty much ideal, and I’m not sure what benefit the new one has.
Source: https://answers.launchpad.net/ubuntu/+source/gdm/+question/86506