Found at http://www.prestonmoore.com/archives/51-Asterisk-Freebx-on-Centos-5.3.html
Install Centos 5.3
yum update && reboot
wget http://nzlinux.com/wp-content/uploads/files/centos53pbx.sh
nano centos53pbx.sh # change "passw0rd" and IP address
sh centos53pbx.sh
Finished