[Samba] our windows APW can't add printer driver to samba domain member

[DJ]

Hi,

I'm part of an organization managing Windows clients with a couple of 
GNU/Linux servers.
We're having some trouble configuring our Samba 3 Debian Linux server.
It's just a domain member with authentification done with a Windows AD 
Domain Controller, it's supposed to be our new print server.
The printers have already been set up for lprng.
Whenever trying to add a driver to any printer throught the Add Printer 
Driver Wizard on a Windows client, with a username part of
the 'print admin' group, we get this error message:
"Unable to install HP Lazerjet 4000 Series PCL 6, Windows 2000, Intel 
Driver. Access denied"

All our printers are in /etc/printcap already.
There are no entries for any of them in smb.conf

gutenberg:/var/log/samba# testparm
Load smb config files from /etc/samba/smb.conf
Processing section "[printers]"
Processing section "[print$]"
Loaded services file OK.
Server role: ROLE_DOMAIN_MEMBER
Press enter to see a dump of your service definitions

# Global parameters
[global]
        workgroup = MYGROUP
        realm = MYGROUP.MYREALM
        server string = %h server (Samba %v)
        security = ADS
        password server = PWDSERV
        syslog = 3
        log file = /var/log/samba/log.%m
        max log size = 1000
        dns proxy = No
        panic action = /usr/share/samba/panic-action %d
        idmap uid = 10000-20000
        idmap gid = 10000-20000
        printer admin = root, @MYGROUP\Staff

[printers]
        comment = All Printers
        path = /var/spool/smbprint
        create mask = 0700
        printable = Yes
        use client driver = Yes
        browseable = No

[print$]
        comment = Printer Drivers
        path = /var/lib/samba/printers
        write list = root, @MYGROUP\Staff
        create mask = 0700
gutenberg:/var/log/samba#

We've read and re-read the chapters/section apropos in the doc, but 
can't find what we're doing wrong.

Help or advice would be greatly appriciated :)
DJ