If you have SELinux enabled you may see an error message in your apache logs like this (even if you'd configured mod_proxy properly):
Tue Jan 29 17:44:03 2008] [error] (13)Permission denied: proxy: HTTP: attempt to connect to 127.0.0.1:8081 (localhost) failed [Tue Jan 29 17:44:03 2008] [error] ap_proxy_connect_backend disabling worker for (localhost)
To enable apache to send network requests you need to run this command:
setsebool httpd_can_network_connect true