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.

  Multiserver Setup With Dedicated Web, Email, DNS & MySQL Database Servers On Debian Squeeze With ISPConfig 3