Comment 40 for bug 516520

OK. Not sure how I managed to do this, but the problem (9.10 > 10.04) machine is now working again. I modified the /etc/apt/custom.conf file to:
===
# GDM Configuration Customization file.
#
# This file is the appropriate place for specifying your customizations to the
# GDM configuration. If you run gdmsetup, it will automatically edit this
# file for you and will cause the daemon and any running GDM GUI programs to
# automatically update with the new configuration. Not all configuration
# options are supported by gdmsetup, so to modify some values it may be
# necessary to modify this file directly by hand.
#
# Older versions of GDM used the "gdm.conf" file for configuration. If your
# system has an old gdm.conf file on the system, it will be used instead of
# this file - so changes made to this file will not take effect. Consider
# migrating your configuration to this file and removing the gdm.conf file.
#
# To hand-edit this file, simply add or modify the key=value combination in
# the appropriate section in the template below. Refer to the comments in the
# /usr/share/gdm/defaults.conf file for information about each option. Also
# refer to the reference documentation.
#
# If you hand edit a GDM configuration file, you should run the following
# command to get the GDM daemon to notice the change. Any running GDM GUI
# programs will also be notified to update with the new configuration.
#
# gdmflexiserver --command="UPDATE_CONFIG <configuration key>"
#
# e.g, the "Enable" key in the "[debug]" section would be "debug/Enable".
#
# You can also run gdm-restart or gdm-safe-restart to cause GDM to restart and
# re-read the new configuration settings. You can also restart GDM by sending
# a HUP or USR1 signal to the daemon. HUP behaves like gdm-restart and causes
# any user session started by GDM to exit immediately while USR1 behaves like
# gdm-safe-restart and will wait until all users log out before restarting GDM.
#
# For full reference documentation see the gnome help browser under
# GNOME|System category. You can also find the docs in HTML form on
# http://www.gnome.org/projects/gdm/
#
# NOTE: Lines that begin with "#" are considered comments.
#
# Have fun!

[daemon]
AutomaticLogin=<username>
AlwaysLoginCurrentSession=false
AutomaticLoginEnable=true
TimedLoginEnable=false
TimedLogin=<username>
TimedLoginDelay=30
DefaultSession=gnome.desktop
===

and now the machine is booting & autologing in. Rebooted twice and the same results - machine boots up and autolog's in w/o issue. Also, System|Administration|Login Screen Unlock is working again after the reboots. I'll see if I can break it again by setting the login to 'show the screen for choosing who will log in' v 'Log in as <username> automatically'. In the interim, can other users having the same isse advise if they: 1) have upgraded from 9.10 to 10.04, have an '/etc/gdm/custom.conf' file on their system?