I fire up sudo nc -l 127.0.0.1 80. There's no errors. I then try telnet 127.0.0.1 80. It won't connect.
Same thing with port 81 works fine.
As far as I know, there's no firewalls running.
https://img.skitch.com/20120228-t8h98sccsde3w1jnf2tb55xdny.png shows the commands I'm using. (the numbers are the order in which the commands are run)
I'm on OS X 10.7.3.
Best Answer
FINALLY figured it out. I had installed Pow (http://pow.cx). It runs this script: http://get.pow.cx. The script installs some firewall rules. Even though I had disabled pow, the firewall rules were still in place. Running the pow uninstall script fixed it.