The gdm screen in lucid does not show the original user

What is the user name of your account? Open a terminal window and type this command followed by ENTER: whoami
If the name is over eight characters, contains characters other than letters and numbers, you may be experiencing bug 476811.

Hi, thanks for the reply. The user name is sean - so I'd already ruled out that bug. (BTW the user name is also sitting there all the time in the upper right-hand corner, if you haven't removed or relocated the session indicator applet - also as mentioned I have to type in the user name to log in!)

When I installed Lucid I kept my home partition but deleted all my config files before installing with the exception of .mozilla and .thunderbird (since for the first time ever I liked the look of the default install and didn't to keep want my heavily customized setup)

I installed as usual, filling out my name, etc when asked, and as usual the installer automatically grabbed my first name as the user name. Until I added the second user I'd just assumed that the developers had decided to ditch the face browser in the gdm for Lucid. I've tried looking through the files in /etc/gdm and /usr/share/gdm but I didn't see anything that refers to the users.

When I go to Users and Groups in the admin menu my own account is labeled as 'custom' but it's basically an administrator account - I can install software, etc using sudo as usual with the password I set during the install, so I assume the user settings are the default ones. I could create a second account for myself as a normal desktop user but then I'd have to do a separate login to check for updates, etc.

In the dialog for changing login window settings under the admin menu, if I choose the 'log in automatically' option, it only lets me choose the second user that I've added for automatic login.

Here's some more info which is a bit surprising: I thought I'd try making the second account into an admin account, and using it to change my own to a desktop user and back again, to see if that made any difference. However, using the other account, when I open the Users and Groups dialog, the original account does not even show up!
So somehow, the system is not seeing the original account, and I'm not really sure why.

Is this a relatively new installation of 10.04?

Ah, well that depends on your definition of relatively! I did a clean install when the final release version came out on the 29th of April. So it's as old as it could be, in that sense, but still pretty new in the larger sense.
More seriously though, I had earlier upgraded from Karmic to Lucid Beta2 (once support for ATI cards was sorted out), but did a clean install after the release version came out. I have /home on a separate partition, but as I mentioned, I deleted all of the settings folders before installing, so that I could see what the default setup looks like. Until Thursday the only user account I had on the machine was my own - I installed keeping the existing /home/sean folder on its own partition, and sticking with sean as the user name during the install. On Thursday I added the second user account (so my girlfriend can log in to play SuperTux...)

I should add that I'm quite happy to do troubleshooting on my own - so basically what I'm looking for is to find out where the system stores information about the list of users - the user list for the gdm would be a start, but it since my account doesn't show up in user management when I'm logged in as someone else, it would probably be helpful to know more generally where this information lives.
I suppose it would be a useful security feature to have the account with admin privileges harder to discover but I don't recall this being the case on previous Ubuntu versions, so I suspect something didn't happen the way it should have when I installed.

All user information is stored in /etc/passwd (http://tldp.org/LDP/lame/LAME/linux-admin-made-easy/shadow-file-formats.html) and passwords are stored in /etc/shadow. Group membership is in /etc/group. You can quickly query the passwd file with "getent passwd sean". And you can easily get your group membership with the "id" command when running it as "sean".

These are the only things I know of that would prevent an account from showing up on the login screen:
  * Bug 46811
  * The user name is listed in the Greeter/Exclude line in /etc/gdm/custom.conf

Here is something to try. Create /etc/gdm/custom.conf that contains:

[daemon]
[security]
[xdmcp]
[greeter]
Include=sean
[chooser]
[debug]

Then reboot and see if the user shows up.

Look at /etc/gdm/gdm.schemas for other options that are available in custom.conf.

Thanks for the suggestions. I tried your last suggestion, but with no luck. My user account shows up in both /etc/passwd and /etc/group. My account is in /etc/passwd with UID 999, and the new account I added has UID 1000. The custom.conf file for the gdm is pretty simplistic - I have only the following:

[daemon]
timedloginenable = False
automaticlogin = None
automaticloginenable = False
timedlogindelay = 30
defaultsession = gnome
timedlogin = None

What seems a bit odd to me is that my user account was created during the install as a 'custom' account - I have admin privileges but do not belong to several of the groups that a normal desktop user belongs to - here are the contents of the passwd file:

root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:sean,jana
tty:x:5:
disk:x:6:
lp:x:7:
mail:x:8:
news:x:9:
uucp:x:10:
man:x:12:
proxy:x:13:
kmem:x:15:
dialout:x:20:sean,jana
fax:x:21:jana
voice:x:22:
cdrom:x:24:sean,jana
floppy:x:25:jana
tape:x:26:jana
sudo:x:27:
audio:x:29:pulse,sean
dip:x:30:jana
www-data:x:33:
backup:x:34:
operator:x:37:
list:x:38:
irc:x:39:
src:x:40:
gnats:x:41:
shadow:x:42:
utmp:x:43:
video:x:44:jana
sasl:x:45:
plugdev:x:46:sean,jana
staff:x:50:
games:x:60:
users:x:100:
nogroup:x:65534:
libuuid:x:101:
crontab:x:102:
syslog:x:103:
fuse:x:104:jana
lpadmin:x:105:sean
ssl-cert:x:106:
messagebus:x:107:
mlocate:x:108:
ssh:x:109:
avahi-autoipd:x:110:
avahi:x:111:
netdev:x:112:
couchdb:x:113:
haldaemon:x:114:
pulse:x:115:
pulse-access:x:116:
rtkit:x:117:
saned:x:118:
admin:x:119:sean
gdm:x:120:
nopasswdlogin:x:121:
sean:x:999:
sambashare:x:122:sean
jana:x:1000:
guest:x:123:

I might try changing my account from custom to admin to see if it makes a difference.
As a workaround I've used the gdm2setup program to disable the face browser, so that at least each user is treated the same!

Thanks for your help.

Brilliant! That did the trick. I guess the trouble may have been caused by keeping my old /home/sean on a separate partition and using it in the install. Since this is my primary machine I might hold off on trying to reproduce the glitch - reinstalling all the additional packages that I use makes it into a few hours' work.
Thanks for all the help! I think I have a bit better idea of how the user accounts work now.

Thanks Erik Meitner, that solved my question.