We're running a cluster, designed before I had much experience with zimbra; here's some background fwiw before the real question at the end:
Release 5.0.18_GA_3011.RHEL5_20090707164432 CentOS5 FOSS edition
2 ldap (master/rep)
The separate proxy system is redundant, having decided to just run the proxies on the mtas, currently with dns balancing, eventually probably a real load balancer.
store01 is the logger/admin system per directions, but didn't quite realize the central administrative role it played at the time.
Having gained a little experience running this cluster with a couple of smaller domains, I'd like to reconfigure it a little before we start migrating en-mass:
1. move the existing mailboxes off store01 and rename it to "zadmin" or something like that
2. remove proxy functions from proxy and turn it into a new store01
There are a couple of other issues with the underlying storage system that make it desirable to completely rebuild the mailstores, but the fundamental question is how do you remove a component system or service from a cluster? Adding seems pretty straightforward...