Description of problem: nagios-plugins are not built for FC3, while nagios is. Additional info: Dag's packages from http://dag.wieers.com/packages/nagios/ and http://dag.wieers.com/packages/nagios-plugins/ are older but work.
Time to upgrade. Sorry but this may never get done for FC3 at least. Check the EPEL stuff when it comes out. The EPEL-4 packages should match with fc3 very well.