hi,

1)a NT4 PDC  10.0.1.8/8 (londres)
2)a NT4 BDC  10.0.1.6/8 (paris)
3)a client smb 3.0.20 mandrake 2006 rpm, ifconfig ->  inet addr:10.1.29.32  Bcast:10.127.255.255  Mask:255.128.0.0 (issus)

try :
net rpc join -s /etc/samba/smb1/smb.conf.smb1 -UAdministrateur -S paris -d 1 -n ISSUS
[2006/11/09 17:27:36, 1] utils/net_rpc.c:run_rpc_command(140)
  rpc command function failed! (NT_STATUS_NO_TRUST_SAM_ACCOUNT)
Password:
Creation of workstation account failed
Unable to join domain IPBS.

idem with -S londres.

Change the netmask of issus -> Bcast:10.255.255.255  Mask:255.0.0.0 
then it works perfectly!

ROOT>> net rpc join -s /etc/samba/smb1/smb.conf.smb1 -UAdministrateur -S londres -d 1 -n ISSUS
[2006/11/09 17:27:49, 1] utils/net_rpc.c:run_rpc_command(140)
  rpc command function failed! (NT_STATUS_NO_TRUST_SAM_ACCOUNT)
Password:
Joined domain IPBS.

By the way the nmblookup command with a different CIDR between machine failed with node not with IP:

nmblookup -s /etc/samba/smb1/smb.conf.smb1 -W IPBS -n issus paris
querying paris on 10.127.255.255
name_query failed to find name paris

nmblookup -s /etc/samba/smb1/smb.conf.smb1 -W IPBS -n issus -A 10.0.1.6
Looking up status of 10.0.1.6
        PARIS           <00> -         M <ACTIVE>
        IPBS            <00> - <GROUP> M <ACTIVE>
        IPBS            <1c> - <GROUP> M <ACTIVE>
        PARIS           <20> -         M <ACTIVE>
        WINDOWS         <20> -         M <ACTIVE>
        IPBS            <1e> - <GROUP> M <ACTIVE>
        PARIS           <03> -         M <ACTIVE>
        PARIS           <01> -         M <ACTIVE>

        MAC Address = 00-60-08-97-B4-54

I can do more test if necessary.