I struggled for a while trying to get video chatting to work with iChat. I tried disabling the OS X firewall and enabling port forwarding on the router, but nothing worked.
Finally I discovered the solution: disable the SPI Firewall on my router (a Netgear WGR614).
To do this, log into the router, choose "WAN Setup" from the left menu, check "Disable SPI Firewall," and click "Apply."
