Just to chime in here and say that I have Zimbra 8 running on Debian 6 at the moment and while the installation process was a clusterf#@% at times, mainly because I didn't completely remove dovecot, it all went fairly smoothly up to this point. Now I only have the pop3 and smtp running with the web interface but that's a great start.
The main issues I'm having now are with the web admin interface and then the zimbra collab features but I'm sure I'll be able to work through those today.
I chronicled some of the fun here:
Migrating away from postfix/dovecot to Zimbra 8 on a Debian 6 server - Netflow Developments
How to add a new domain to a Zimbra install and then a new account to the domain - Netflow Developments
Setting up email forwards in Zimbra with zmprov - Netflow Developments
and will blog while going through the next batch of adventures to get the rest of it up and running.