I know this is an old thread, but I just wanted to say that (while my FOSS install was working) I followed the backup scripts here:
http://wiki.zimbra.com/index.php?tit..._Script_Method
The only thing I changed was that I'm not FTP'ing the backup archive anywhere, but rather simply cp'ing it out to a mounted NTFS network share. It works/worked perfectly and has been tested with a recent restore, so I know it's solid.