After installation of Ubuntu 9.10 Karmic Koala beta I found out that the GDM was completely changed. And the main part - it cannot be themed anymore. Which I didn't like at all. Thanks god they kept the old GDM in repositories as well. It's named gdm-2.20. However, there's a bug in that package which has to be fixed manually afterwards.
First we install the "old" GDM.
We'll be prompted for the default gdm, choose gdm-2.20. Now the installation is finished, however we need to fix a bug in GDM configuration file.
Find all occurences of /usr/X11R6/bin/X and change it to /usr/bin/X. Now we're finished and can run the GDM.
If you want to have the ability to restart your X server with the magic key combination ctrl+alt+backspace in Jaunty and later, add the following section to your /etc/X11/xorg.conf.
Section "ServerFlags" Option "DontZap" "off" EndSection
In order to disable the automatic window raising and restore the Jaunty notification style, use the following command.