I didn't find the complete answer here and wound up digging through userland.portforward before also finding the answer ont he TLS pages. But for future reference, here's how to do it in a terminal session. I'll figure out how to do with with a shell script later (and repost):

1. in the terminal, type: sudo /usr/sbin/sysctl -w net.inet.ip.forwarding=1
2. authenticate when asked
3. type: sudo /sbin/ipfw add 102 fwd 127.0.0.1,8080 tcp from any to any 80 in

Clark