Since FC3 is going to become RHEL4, I was suprised that Nagios was not included as a package. "Nagios is a host and service monitor designed to inform you of network problems before your clients, end-users or managers do." -- http://www.nagios.org/about.php
I don't think this will be added to Fedora Core at this point. Adding to Fedora Extras/Fedora.us would probably be the best first step.