Howtoforge posted a tutorial about installing Icinga on Debian Squeeze

Icinga is an enterprise grade open source monitoring system which keeps watch over networks and any conceivable network resource, notifies the user of errors and recoveries and generates performance data for reporting. It is a fork of Nagios. This tutorial explains how to install Icinga on a Debian Squeeze server to monitor this server and another Debian Squeeze server.

  Server Monitoring With Icinga On Debian Squeeze