If the httpd server process fails to start, check /var/log/httpd/error_log
If you see something like this:
[Tue Nov 29 11:33:15 2011] [error] SSL Library Error: -8181 Certificate has expired [Tue Nov 29 11:33:15 2011] [error] Unable to verify certificate 'Server-Cert'. Add "NSSEnforceValidCerts off" to nss.conf so the server can start until the problem can be resolved.
then the only cure appears to be to remove the "mod_nss" package from the system. Use yum to uninstall it.