I created a user account for myself, and I wanted to use the same uid that I use on our Sun Solaris cluster, which is 5289. When I logged in through gdm, I would see the "Enlightenment starting" progress bar. Then I would see the desktop and my mouse cursor, but nothing else. The taskbar, gnome-help-browser and gnome-filemanager apps did not show up. I resolved the problem by dropping the uid of my user account down to 500. Yes, I was very careful to chown -R files in the user directory over when I changed the uid, so that does not appear to be the problem.
I use gnome-session (which brings up ghelp and gmc) successfully with a UID of 14881 (above the 5289 indicated in the bug report above) which is supplied via NIS from a Sun Solaris network. Thus I can confirm there is no problem with a NIS UID as high as 14881 starting gnome-session on Red hat 6.2. I hope this helps narrow it down.
I can't get this to happen; maybe send .xsession-errors, maybe it shows why the login fails for this user?