When you mount a samba share without unix extensions enabled, you can set a GID, UID and permissions (on the client machine, at mount time) so you can adjust it to let non-root users use it.
Mounting FAT works this way as well. But there is a big difference. Where new files on a FAT file system are created according to the permissions you set at mount-time, new files on a SAMBA share have their permissions determined by the umask. However, when you unmount and remount the share, the permissions are changed to what they were set to at mount time.
This is very annoying behavior, because when you have files that belong to root:smbusers and you copy a file, it still belongs to root:smbusers, but when your umask is 0022, it will no longer be group writable and it has become a read only file.
I think this is a bug in the SMBFS/CIFS file system driver.