Howtoforge posted a tutorial about setting up a Multiserver Setup with ISPConfig 3 and Debian 6
This tutorial describes the installation of an ISPConfig 3 multiserver setup with dedicated web, email, database and two DNS servers all managed trough a single ISPConfig 3 control panel. The setup described below uses five servers and can be extended easily to to a higher number of servers by just adding more servers. E.g. if you want to have two mailservers, do the setup steps from chapter 2 on both of these servers. If you want to set up more web servers, then install ISPConfig on all other web servers in expert mode except of the first one.