I've been doing a live sync for quite sometime now using a modified version of the technique the Parent has posted.
Basically, I start off like the Parent, with a server configured identically to my Live server.
I then have a script that Rsyncs the store,index,zimlets-depoyed, and db/data directories on our Live running server.
Now, this creates the issue of out-of-sync logs since there is a little bit of a time delay in backing up the database. I resolve this by running a full dump and restore of my Zimbra and Mailbox databases.
The end result is a cold-standby that is updated hourly. I can easily test the server content by logging in using it's own IP address. I use a script similar to the Parent to switch from COLD to HOT in case of main server failure. When the Main server is repaired or replaced, I run the same procedure in reverse. It has saved me now on 3 separate occasions. The most data I risk losing is one hours worth of email (assuming the Main server dies just before the hourly script runs).
I started doing this under Ubuntu 6.04x86 and Zimbra 6. I now do this with Ubuntu 8.04lts x64 and Zimbra 6.0.7NE.
I of course have nightly backups being performed using Zimbra's built in functionality, but should something bad happen, the Cold Standby is an immediate and easier option to get back to operational status.