The latest version of GDM has been uploaded into Ubuntu Karmic and is under active testing. There are few issues to be ironed out before we can have a fully accessible login with orca screen reader and magnifier. Until then I advice the blind gnu/Linux users to use GDM timed login.
Add the following lines to your /etc/gdm/custom.conf file to enable GDM timed login. Replace 'user' with your username.
Please note that the configuration of GDM has moved away from previously single configuration file /etc/gdm/gdm.conf . As of now you can't use the graphic gdmsetup to configure GDM.