Comment 2 for bug 237115

I'm having this problem as well, although for me it's not limited to GDM (not sure if it is for the reporter).

auth.log has this entry when the problem is exhibited:
Jul 15 15:20:43 ******** nscd: nss_ldap: could not search LDAP server - Server is unavailable

Symptoms:
A lookup on the current user will fail, resulting in several problems: Not able to lock the desktop (dbus connection seems to fail), not able to use gnome-terminal (it will report the user as "I have no name" and shout "You don't exist, please go away!" at every command)

Our system setup:
libnss-ldap for passwd/shadow/group lookups in Active Directory (Windows Server 2003-R2 / 2008)
libpam-krb5 for authentication

libnss-ldap uses a dedicated user when called as root (rootbinddn/passwd) and when called as mortal user it uses sasl for kerberos authenticated ldap lookups)