Steve,

To answer your questions about the bug I am seeing, I have compiled the following. Look below, there is a significant difference in the client side file permissions when cifs is used to mount the volume and when smbfs is used. This is fully repeatable and the listings below did not require system reboot, just different -t (cifs|smbfs) switches on the mount.

1. I first saw the problem after upgrading a single client to 8.04 (AMD 64 bit). The server was then Xubuntu 7.10 with standard up-to-date samba packages. This was then upgraded to 8.04 Xubuntu and the problem remained. 

2. The definition of the share is:

[TASData]
	comment = TasBooks Data Files
	path = /raiddisk/shared/tasbooks
	valid users = @accounts
	force group = accounts
	read only = No
	acl check permissions = No
	directory mask = 0775
	force directory mode = 00
	browseable = No

3. The fstab is:

//olympus/tasdata	/home/tony/tasdata	cifs	credentials=/etc/samba/tw.pw,uid=1000,gid=1000,auto,rw	0	0

4. The server side listing is:

-rw-rw-r-- 1 tony accounts     3584 2008-04-28 11:56 mt1pmt.b
-rw-rw-r-- 1 tony accounts     3584 2008-04-28 11:56 mt2pmt.b
-rw-rw-r-- 1 tony accounts     3584 2008-04-28 11:56 mt3pmt.b
-rw-rw-r-- 1 tony accounts     3072 2008-04-28 11:56 mt4pmt.b
-rw-rw-r-- 1 tony accounts     3072 2008-04-28 11:56 mtabacs.b
-rw-rw-r-- 1 tony accounts     6144 2008-04-28 11:56 mtabank.b
-rw-rw-r-- 1 tony accounts     3072 2008-04-28 11:56 mtabia.b
-rw-rw-r-- 1 tony accounts    26624 2008-04-28 11:56 mtabud.b
-rw-rw-r-- 1 tony accounts     3072 2008-04-28 11:56 mtacadv.b
-rw-rw-r-- 1 tony accounts     7680 2008-05-05 14:43 mtaccat.b
-rw-rw-r-- 1 tony accounts    10240 2008-05-15 22:13 mtacent.b
<truncated - all other files have the same permissions>

5. The client side listing (when mounted as CIFS) is:
-rw-rw-r-- 1 tony tony    3584 2008-04-28 11:56 mt1pmt.b
-rw-rw-r-- 1 tony tony    3584 2008-04-28 11:56 mt2pmt.b
-rw-rw-r-- 1 tony tony    3584 2008-04-28 11:56 mt3pmt.b
-rw-rw-r-- 1 tony tony    3072 2008-04-28 11:56 mt4pmt.b
-rw-rw-r-- 1 tony tony    3072 2008-04-28 11:56 mtabacs.b
-rw-rw-r-- 1 tony tony    6144 2008-04-28 11:56 mtabank.b
-rw-rw-r-- 1 tony tony    3072 2008-04-28 11:56 mtabia.b
-rw-rw-r-- 1 tony tony   26624 2008-04-28 11:56 mtabud.b
-rw-rw-r-- 1 tony tony    3072 2008-04-28 11:56 mtacadv.b
-rw-rw-r-- 1 tony tony    7680 2008-05-05 14:43 mtaccat.b
-rw-rw-r-- 1 tony tony   10240 2008-05-15 22:13 mtacent.b

The client listing when mounted as smbfs is:
-rwxr-xr-x 1 tony tony    3584 2008-04-28 11:56 mt1pmt.b
-rwxr-xr-x 1 tony tony    3584 2008-04-28 11:56 mt2pmt.b
-rwxr-xr-x 1 tony tony    3584 2008-04-28 11:56 mt3pmt.b
-rwxr-xr-x 1 tony tony    3072 2008-04-28 11:56 mt4pmt.b
-rwxr-xr-x 1 tony tony    3072 2008-04-28 11:56 mtabacs.b
-rwxr-xr-x 1 tony tony    6144 2008-04-28 11:56 mtabank.b
-rwxr-xr-x 1 tony tony    3072 2008-04-28 11:56 mtabia.b
-rwxr-xr-x 1 tony tony   26624 2008-04-28 11:56 mtabud.b
-rwxr-xr-x 1 tony tony    3072 2008-04-28 11:56 mtacadv.b
-rwxr-xr-x 1 tony tony    7680 2008-05-05 14:43 mtaccat.b
-rwxr-xr-x 1 tony tony   10240 2008-05-15 22:13 mtacent.b


7. The underlying problem is when the closed source TAS Books program tries to open its accounts database. It reports that the database is being used by a maintenance utility. However, you get exactly the same error message if the share is not mounted - so that is not really very useful. I have no problem opening any of these files in a text editor (under cifs), or in saving them, so this is not a simple permissions problem. While it is not really possible to know what the program is doing, I know that it will try and gain exclusive access to the files and my guess is that it is failing at this point.

One thing is certain, now that I have compiled in the smbfs module from samba 3.0.28a, if I mount the share as smbfs, the program works, if I mount it as cifs, it doesn't.