- Turn the built-in Firewall OFF
- If the firewall is on, turn Personal Web Sharing on as well
- Under Firewall settings, add a New Port set to Port 80 (just call it "Airport Internet Sharing" or whatever you'd like), and turn it ON. Basically, Port 80 has to be turned on in the Firewall.
Easily fixable, but a stupid glitch to let get by -- even a simple note of explanation written in the Internet Sharing dialogue would've been enough!

