samba - Jan 2007 - failing to create a machine trust account

Hi,

I'm trying to create a machine trust account using samba 3.0.23d. I tried 
on-the-fly creation but this failed and I found this in the log:

[2007/01/07 23:03:04, 2, pid=28573, effective(1000, 1000), real(1000, 0)] 
rpc_server/srv_samr_nt.c:_samr_lookup_domain(2797)
  Returning domain sid for domain EST -> 
S-1-5-21-1152323856-1746261438-3327382661
[2007/01/07 23:03:04, 2, pid=28573, effective(1000, 1000), real(1000, 0)] 
rpc_server/srv_samr_nt.c:_samr_lookup_domain(2797)
  Returning domain sid for domain EST -> 
S-1-5-21-1152323856-1746261438-3327382661
useradd: unable to lock password file
[2007/01/07 23:03:04, 0, pid=28573, effective(1000, 1000), real(1000, 0)] 
passdb/pdb_interface.c:pdb_default_create_user(368)
  _samr_create_user: Running the command `/usr/sbin/useradd -d /dev/null -g 
100 -s /bin/false tbrownxp$' gave 1

Why is useradd unable to lock the password file?

Then I tried to create the machine trust account manually like this:
/usr/sbin/useradd -d /dev/null -g 100 -s /bin/false tbrownxp$

That adds tbrownxp$ to /etc/passwd. Then I do this: 
smbpasswd -c /usr/local/samba/lib/smb.conf -m tbrownxp

That fails with this error: 
Failed to find entry for user tbrownxp$.
Failed to modify password entry for user tbrownxp$

Why is smbpasswd not finding tbrownxp$ when I add a machine trust account?

Thanks,
Tom