For load balancing check the multi-server installation guide.
http://www.zimbra.com/docs/os/latest...1.html#1074736
You can't do round robin or load based load balancing but you can break up your users into different mail stores and use that to break up your user load. Of course that means that you need failover for each mail store.
We plan to use VMware to keep hardware failover out of the equation - at least as far as our Zimbra installation is concerned.