This comes up a lot. In addition to the reverse proxy, you can also:
-- redirect port 80 to port 3000
iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3000
-- use authbind to grant port 80 access to an unprivileged user
http://manpages.ubuntu.com/manpages/hardy/man1/authbind.1.ht...