There are a couple of choices. One is to install gdm.2.20 (the old gdm)
sudo apt-get install gdm-2.20
sed ’s|X11R6/||’ gdm.conf >/tmp/gdm.conf
mv /tmp/gdm.conf .
During the install of gdm-2.20 you'll be asked to choose which one to run. The remaining commands edit the file to replace instances of /X11R6/bin with just /bin ( a bug filed with launchpad regarding installing the previous version. Anyway, after the above you're back to being able to use favorite old themes from gnomelook, etc. And the old login window options
The other choice is to learn to configure the new gdm. There is information out there. For example this blog: http://lionlix.wordpress.com/2009/10...-login-screen/
This method gives limited results like changing the background image