Well, as the original problem was in January I'm guessing it did get fixed. You don't mention which version of Zimbra you're running? Have you also done a search of bugzilla to see if the bug was fixed?
I'm running Zimbra 4.0.0. I was hoping marcmac might followup to my comment, as he said there was a bug about it earlier in the thread.
I've tried to use bugzilla.zimbra.com. I couldn't find the bug marcmac was referring to. When using the simple search for a bug, I had 200+ hits, but when I tried to use the advanced search to narrow that down, I couldn't get any hits. I've never understood bugzilla.
i haven't retried my original proxy rewrite stuff which would have shared an existing vhost on a /webmail/ subdirectory, so to speak, but if you search the wiki for apache you'll find a reverse proxy method that i currently use and it works great. if you got your zimbra webmail running on 10001, you point a vhost at that port. it does mean running a seperate apache vhost and an additional dns entry, but does mean you can get the webmail running on port 80 alongside your regular websites.