After numerous problems, I recently returned one box running Mandriva to a Ubuntu Karmic box.

As I always do, I keep the /home partition intact. This may or may not be important.
As I always have before, I assign the same users to the same folders on the /home partition, to ensure everyone has immediate access to their files and settings.
This time, it didn't go to plan.

I created a temporary user so I could set up everyone else's user accounts.
Using Gnome, I used System->Administration->Users and Groups
I created my normal user.
It wouldn't let me assign it to the already existing folder for my user, so I assigned it to /home/temp, later to be removed - in theory.
I told it to create the user. It claimed sucess, and returned to the user list.
Where nothing had changed. Only the temporary user and root were listed.
I installed KUser.
Running not as root, it of course, complains.
Running as root, it doesn't, and happily lists all users. It says my account exists, is not disabled, and even let me change the home folder to the correct directory.
However, Gnome's Users and Groups dialog still reports it nonexistant, and won't let me create a new one with the same name or ID, of course.
The GDM login screen's face browser also doesn't list it.
This is a problem - the other users who use this computer regularly use that face browser because (inconveniently) they can't remember their own username. I'm not going to try to get them to remember, because as usual I'll be ignored.
So it'd be very much appreciated if someone could help diagnose the problem.

(As I type this, KDM is being installed in an attempt to see if it can 'see' my otherwise nonexistant user. Since KUser can, I'm optimistic.)

Can you log in with the account at all ? ( Select "other" from GDM )

It appears to be so, which is alright for me - but not for them. Which is why I've been trying to resolve the user's apparent refusal to show up.

Although this probably isn't of any help have you tried: man invisible ????

I have been comparing my /etc/pam.d/kdm and /etc/pam.d/gdm files and they are basically the same except that gdm uses the pam_gnome_keyring.so - can you say the same of yours?

No manual entry for invisible, by the way.

Indeed, my /etc/pam.d/gdm file does use the pam_gnome_keyring.so

KDM, however, also seems to suffer from the inability to display an entry for additional users, so for now I've returned to GDM.

Brave the man who takes the path less trodden on buggy systems!

Complex systems -- especially ones with a leading edge design goal -- are prone to bugs, many of which escape detection in pre-release testing and so are only identified "in the wild". They are then classified according to impact and fixed in impact order. The effect is that a) bugs are more likely in seldom-used code or in code used with unusual data and b) such bugs are not fixed quickly.

(Ubuntu Karmic) + (user with home directory other than /home/$USERNAME) = minefield

What bothers me though is that if I try to create a user linking to an already existant directory, Gnome won't let you do it - which is a major pest on systems which are meant to have more than one user.
KUser solves that, but the users it creates don't show up either. More of a pest.
And the User management in the KDE system settings... crashes.
I'm starting to wonder if Jaunty wasn't a better system then Karmic.

I am using Ubuntu only as a single user machine (well, real existing user at least) so I am not sure if this will work. But have you considered to remove that user as root (using the console) and then recreate him? Of course you should backup the user's data first. Afterwards you can copy the user's data back into his new home directory.
This could be automated with a script.

The problem still remains that the newly created user doesn't appear in the face browser - that's the core problem for me here at the moment. Given that most of the other users still think 'the windows way' a lot, and just want it to work, they're used to things like XP's Welcome screen, and want it to be just the same way. I don't mind that, but the system's refusal to cooperate and show the users is a major hinderance.