A recent Apple support note points to the issue:
When attempting to use smbpasswd to change a user's password in Terminal, the command does not work. Instead, the message "smbpasswd: Command not found" appears in Terminal. Resolution To change a user account password in Lion, open the Users and Groups pane of System Preferences. The Mac must be bound to same domain the user belongs to. Additional Information The smbpasswd command line utility is part of Samba, which is not included with Lion or Lion Server.
If you ever want to find the list list of acceptable SMBX commands, type smbutil in Terminal.
Apple started bundling Samba in Mac OS X Jaguar, v.10.2, to support Microsoft's SMB (server message block) file and directory services. With the introduction of OS X Lion, it switched to an in-house software, which supports the MS-SMB2 protocol released in 2007. The previous Mac Samba implementation supported SMB1.
The switch hasn't been pain free, with a rash early complaints cropping up with NAS (network-attached storage) units that relied on SMB for file services. There still are plenty of file-sharing and access complaints on Apple's discussion boards. Often there will be no problem with Windows CIFS/Samba or the open-source Samba under Linuz, but when we get to SMBX, there's a different story.
We can hope for improvement in Mountain Lion.