FWIW, I put Nagios in /home/nagios/html, and use an apache VirtualHost
directive in a conf file in /etc/httpd/conf.d

Interesting.  I don't know much about these virtual hosts, and will look more into it.

I assume this is where I need to look?

http://httpd.apache.org/docs/2.0/vhosts/

and

http://www.devshed.com/c/a/Apache/Configuring-and-Using-Virtual-Hosts-in-Apache/

thx, John!