In the past I've worked quite a lot with Red Hat Cluster Suite and other Linux based active/passive failover clustering technologies. As technology has moved on, so have the available options for cluster software. Pacemaker + Corosync are two of these fantastic technologies.
Ante Karamatić recently sent a mail to the Ubuntu server list, to ask for help testing the latest packages... If you are interested in cluster technologies in Ubuntu then this is a great opportunity to get involved, all instructions are provided on the wiki page so get testing!
See the UDS Lucid blueprint for the cluster stack for more info on the progress: https://blueprints.edge.launchpad.net/ubuntu/+spec/server-lucid-cluster-stack