The problem is that the Samba machine accounts are created as disabled. It seems the reason for this is a bug in the CentOS 4.3 Samba packages, because there were no problems with SME 7 Beta/Pre editions which were CentOS 4.2 based - also look at
http://www.gatago.com/linux/samba/19616486.html.
Fortunately the solution is simple. After the machine account is automaticly generated as disabled, log to the SME machine as root and type 'smbpasswd -a -m client_machine_name$' (you can see the correct machine name by doing 'cat /etc/samba/smbpasswd'). This should convert the disabled account to enabled. Then on the client machine, also as root, type 'net rpc join -U admin' and type the SME admin password. The client machine should now be joined to the windows domain.
I've tested this both with Openfiler 2.1 Beta1 NAS (where I had to try joining the SME windows domain twice in order to see the domain users and groups, I don't know why) and with a CentOS 4.3 workstation. There were no problems accessing the Openfiler Samba shares or loging on the CentOS workstation with the domain users (configuring CentOS 4.3 for this was more longwinded then I expected, but it was not a big problem).
Hopefully this will help everybody that was trying to authenticate Linux machines against a SME box.
Regards,
Goran