This happens now on my system as well. It worked before, so my bet is that one of the *.cfg's got damaged. But which one...? When I do a configcheck, I only get:
/usr/nagios/bin/nagios -v /etc/nagios/nagios.cfg
Copyright (c) 1999-2005 Ethan Galstad (www.nagios.org)
Last Modified: 02-09-2005
Reading configuration data...
This is on a Gentoo system with nagios-2 b2