RULE ONE: Before you can add a user to the smbpasswd password file, that user must have an UNIX/Linux account (typically stored in /etc/passwd) on the system hosting the Samba service.
And that's what I was missing. You can't add a Samba user unless the posix user account exists first. This applies to Samba implementations with a LDAP "back end" just as much as it applies to traditional Samba implementations. It never occurred to me that this would be the case. On a Windows Active Directory server, you don't have to create a local user account as well as creating a user account in AD, the LDAP database. On NT 4.0 Servers - which Samba was originally developed to imitate - things are different, of course. But in February 2000 Windows moved on: Samba has still to catch up eight years later.
Suddenly the scales have been lifted from my eyes. Fedora Directory Services, and indeed other Linux LDAP implementations, are at best "bolt ons" for Samba. LDAP does not fundamentally change the way Samba operates, and emphatically Samba + LDAP does not equal AD.
Crucially, Samba cannot integrate LDAP with the Linux file system. On a Windows server, you can create a group in AD and then give that group access to a folder. Samba 4 will have its own LDAP directory that will allow it to be an LDAP server for AD clients. However, there will still be no integration with the file system. As a recent article in Linux Magazine puts it:
On a more somber note, Samba4 lacks great integration with the POSIX system on which it sits. It cannot map NT ACLs into POSIX ACLs, it requires users and groups to be added to Linux as well as to its internal ldb database.
So why bother implementing Samba (3) with LDAP at all? For the moment I can't think of a single good reason. Apparently a LDAP back end provides better scalability and performance. But any organization needing scalability and performance isn't going to be looking at hacking around with Samba and LDAP anyway. They are going to look to Active Directory, and the product that Active Directory set out to destroy - Novell Netware. I'm suddenly very interested in how OES integrates with the Linux file system - if indeed it does.
My illusions shattered, I got Samba and LDAP working quite quickly. It was just a case of creating the accounts I needed using useradd or system-config-users. This included creating the "Administrator" account (which then allowed me to use pdbedit to set the SID) and my Windows workstation accounts:
/usr/sbin/useradd -n -c "Workstation FDSPC" -M -d /nohome -s /bin/false FDSPC$
I then called "smbpasswd -D 10 -a Administrator" as I did before. This time, however, I got this:smbldap_search_ext: base => [dc=riverside,dc=forensit,dc=com], filter => [(&(uid=Administrator)(objectclass=sambaSamAccount))], scope => 
ldapsam_getsampwnam: Unable to locate user [Administrator] count=0
pdb_set_username: setting username Administrator, was
pdb_set_full_name: setting full name Administrator, was
pdb_set_domain: setting domain RIVERSIDE, was
which makes sense: Samba checks LDAP to see if the accounts exists and creates it if it doesn't. I used smbpasswd again to create the workstation accounts:
/usr/bin/smbpasswd -a -m FDSPC$
This done, I was finally able to add my Windows workstation to my Samba LDAP domain.