Hi,
I don't think I can help you with your problem, but I can describe what worked for me: using Zimbra Network Edition (as far as I know in the Open Source version it is not possible to enable mobile support). So you need Zimbra licenses.
I installed Zimbra with a self-signed certificate first but then went for a cheap certificate bought at godaddy.com because the self-signed certificates are annoying in the browsers...
Tried syncing with E71 only last week. It was only adding the Zimbra server in the mail for exchange application and it worked perfectly!
No, you don't want to go back to exchange 2007
-Bernhard