GNOME upstream worked on a GDM code refactoring some cycles ago, the new version is not used in ubuntu yet, but it's being considered for next jaunty+1 and it would be nice to start giving it testing now.
The upgrade plans are described on https://wiki.ubuntu.com/DesktopTeam/Specs/Jaunty/GdmUpgrade and you can get it a first version in the desktop team ppa now.
To try it add "deb http://ppa.launchpad.net/ubuntu-desktop/ppa/ubuntu intrepid main" to your sources.list and install gdm-new
There is some known issues:
- the new version conflicts with gdm, fast-user-switch-applet and gdm-guest-session, those and ubuntu-desktop will be removed when you install the new gdm version
- the new gdm has a switch user applet but it didn't get the ubuntu changes which happened during the intrepid cycle
- the new gdm will get started directly after installation, that should close your session but you will be switched to gdm, you can select your current user to resume the session use though
- there is no configuration migration nor graphical tools to do changes to the configuration
- the new version might be lacking some of the options you were using
The new version should be mostly working though and let you log in, switch between users, suspend the computer, reboot, etc.
If you do try the new version and have some comments feel free to reply on the Ubuntu desktop mailing list, if the user feedback is good the new version will be uploaded to universe.
Source: Sebastien Bacher
For more information about the new GDM, visit http://live.gnome.org/GDM/NewDesign