smb.conf (slightly edited):

[global]
             client lanman auth = no
             client NTLMv2 auth = yes
             client plaintext auth = no
             client use spnego = no
             create mask = 0644
             encrypt passwords = yes
             guest account = smbguest
             kernel change notify = no
             log level = 1
             map archive = no
             name resolve order = wins hosts bcast
             netbios name = server1
             password server = *
             security = domain
             server signing = auto
             server string =
             socket options = TCP_NODELAY SO_KEEPALIVE SO_SNDBUF=32768 
SO_RCVBUF=32768
             strict locking = no
             username map = /usr/local/samba/lib/username.map
             utmp = yes
             wide links = no
             wins server = XXX.XXX.XXX.XXX
             workgroup = OURDOMAIN

[share1]
             browseable = no
             path = /spare/share
             valid users = unixuser1 unixuser2 smbguest
             write list = unixuser2



username.map:

!unixuser2    = domainuser_2 domainuser_3
!unixuser1    = domainuser_1
smbguest      = *



Reproducing the symptom:
- map share1 as any domain user
- in Windows Explorer (Win2k SP4), right click on any file in share1 and 
select 'Properties'
- click 'Security' tab
- file owner is ALWAYS shown as 'smbguest (SERVER1\unixfileowner)'
- file group is shown (correctly) as 'unixfilegroup (SERVER1\unixfilegroup)'




Further Information:

I have discovered that if 'unixfileowner = unixfileowner' is added to 
username.map the file owner is shown correctly as 'unixfileowner (SERVER1
\unixfileowner)'.

Samba 2.2.8a displays the file owner correctly without modifying username.map.


Speculation:
After adding a few well placed debugs it appears that when Samba is populating 
the Name field of the Security tab the function 'local_lookup_name' calls the 
function 'map_username' with 'unixfileowner' as the argument. In the unmodified 
username.map file 'unixfileowner' can never match any RHS and consequently 
falls through to the wildcard entry.