iPhone 4 - FaceTime and Firewalls
Tuesday, June 29, 2010 at 3:53PM 
If you have been having trouble using FaceTime with your iPhone 4, it could be a firewall problem. I am using a Linksys router, and mine 'just worked'. If you are having issues, open up these ports on your router.
From Apple's FaceTime support document, they say:
If the Wi-Fi network router that you are connected to uses a firewall or security software to restrict Internet access, contact the network administrator and reference this technical article. To use FaceTime on a restricted Wi-Fi network, port forwarding must be enabled for ports 53, 80, 443, 4080, 5223, and 16393-16472 (UDP).
The Wi-Fi network administrator can refer to their router, firewall, or security software documentation for information on configuring port forwarding.
If you encounter issues using a Wi-Fi network, use standard Wi-Fi network troubleshooting to resolve interference and other issues.
To test, give Apple a call and test FaceTime with them. 1-888-FACETIME (not a joke, they really have reps to FaceTime with you, during their normal business hours).