IP fault tolerance on Debian Squeeze couldn’t be easier than with UCARP!:

• Install UCARP on all nodes:

  apt-get install ucarp

• Edit /etc/network/interfaces on all nodes, replacing <CLUSTER IP> and <PASSWORD> as appropriate:

  iface eth0 inet static
  ...
  ucarp-vid 3
  ucarp-vip <CLUSTER IP>
  ucarp-password <PASSWORD>
  ucarp-xparam -n
  iface eth0:ucarp inet static
  address <CLUSTER IP>
  netmask 255.255.255.255

• Reboot, and you’re done!  <CLUSTER IP> will now be available from one of the nodes in your cluster at all times!

A few notes:

• All nodes in your UCARP cluster must be in the same subnet, and the shared cluster IP must also be in the same subnet.
• UCARP does not do cluster resource management for you, but it does provide hooks.  You will need to write some scripts to handle unmounting/mounting of shared storage, etc.
• UCARP does not detect service failures, only whole-host (or IP stack) failures.  But if you can detect the failure, simply send SIGUSR2 to the ucarp process to force into passive mode, forcing another node to become master.