If you are using Debian lenny/sid and Nagios complains after apt-get install nagios3 with the message: „Error: Could not read object configuration data!“ than one solution could be to add the user www-data to the group www-data.
adduser www-data www-data
Don’t forget to reload the apache process.
For some strange reason this user was not in his default group and so the CGI could not read the file /var/cache/nagios3/objects.cache which is only accessible for user „nagios“ and group „www-data“.
I came across this with strace, following one of the apaches PIDs and reloading the page until the round-robin activated my process and there was some output which I grepped for „nagios“.