Nginx runs under your own user, so it can't use a privileged port. Instead it runs on port 8080. You may want to have it run on port 80, and if so you can add a simple firewall script to forward the port. (This is totally optional.)
cd /Library/LaunchDaemons
sudo curl -O https://raw.github.com/gist/945906/d817356fb5db195c7c4c46fd39f5f5cac6db6e8f/com.coverallcrew.firewall.plist
sudo launchctl load -w com.coverallcrew.firewall.plist