[Samba] Read-only alert for MS Office files on samba share

[Admin machines]

Hi everybody,

We've been running a samba server for some time, which is used both as PDC
and share space.
Recently, we bought some new machines which came with Windows 7 installed. I
struggled to find a solution to make them join the domain, and discovered I
had to upgrade samba for that, which I did.
We are now running version 3.4.9-42 on CentOS5.

Since this upgrade, we have problems with MS Office files that are opened
read-only because already open by "another user".
This happens very often on the Windows 7 machines, quite often on Vista
machines and I did not see it happen (yet at least) on XP machines.

We found a workaround to avoid the problem : instead of double clicking on
the shared file, if we right click on it and click on open, the problem does
not happen, which seems really strange !

When I try to open the file from the Windows machine and it complains the
file is "already in use by another user", I checked on the samba side :
  - The file permissions are correct
  - There is no ~xxx file
  - The file is not open at all according to smbstatus
At the same time, if I try to open the file on a Windows XP machine, I have
no problem at all, whatever network user I use.

Could anybody help ?