Howtoforge published a guide about setting up a load balanced high-availability Apache cluster on Ubuntu 8.04 LTS

This tutorial shows how to set up a two-node Apache web server cluster that provides high-availability. In front of the Apache cluster we create a load balancer that splits up incoming requests between the two Apache nodes. Because we do not want the load balancer to become another "Single Point Of Failure", we must provide high-availability for the load balancer, too. Therefore our load balancer will in fact consist out of two load balancer nodes that monitor each other using heartbeat, and if one load balancer fails, the other takes over silently.

How To Set Up A Loadbalanced High-Availability Apache Cluster On Ubuntu 8.04 LTS