I think that you can exclude usernames from the list on Centos 6 by making their user number less than 500.
That doesn’t help me, as I have no users defined in the /etc/passwd file, and the UIDs are defined in a corporate database that match the employee number.
You can (also) exclude entries by enumerating them on an "Exclude=" line in the greeter section of /etc/gdm/custom.conf .
Not what I had in mind either, but this *may* help.
Alternatively, you can suppress the list totally by changing the default of gdm->simple-greeter->disable_user_list with the gconf editor.
I still like to see my name if I’m the only one that uses the system, and same goes for others.
What I am really looking for is where gdm (or whatever) caches the list of users who have previously logged in to a system. I have tried the brute force approach (grep -R) without success.
Alfred