lightdm and Unity don't recognize user after its UID has been lowered

[Thomas TEMPE]

What I did :
1) edited /etc/passwd by hand. Modified user id for user "thomas" from 1000 to 501
2) modified /etc/passwd by hand. Modified group id for group "thomas" to 20. Removed goup "dialout", whose id was 20.
3) modified the ownership of /home/thomas accordingly

My purpose : sharing the /home filesystem with a different OS (Mac OS X)

Observed problems :
1) in the LightDM login screen, where all users are listed, user "thomas" disappeared. I can still login by choosing the "another user" option and typing "thomas" as "username".
2) in the Unity menubar, next to the "user" icon (top-right corner of the screen), instead of my full name, I see the text "[Invalid UTF-8]". However, my name contains no non-ascii character in /etc/passwd.
3) When entering the "user accounts" configuration window, I can modify other users' accounts, but I cannot select my own anymore. When logging as user "test", user "thomas" is not displayed in the list anymore.

Additionnal informations :
thomas@ubuntu-air:~$ lsb_release -rd
Description: Ubuntu 11.10
Release: 11.10
thomas@ubuntu-air:/etc/lightdm$ tail -n 4 /etc/lightdm/users.conf
[UserAccounts]
minimum-uid=500
hidden-users=nobody nobody4 noaccess test
hidden-shells=/bin/false /usr/sbin/nologin

[Robert Roth (evfool)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. This particular bug has already been reported and is a duplicate of bug #874194, so it is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Feel free to continue to report any other bugs you may find.