My sister recently purchased a new MacBook, and I've been trying to do a video iChat with her for the last few days. It worked once and mysteriously stopped. I know that my router works, because I use it with Amanda. I had to turn on UPnP for it to work reliably, but I finally got things working today. I had turned on UPnP on her router, but things still weren't working. It turns out that there was some "secure" vs. "open" NAT option in the WAN set-up. Turning that option to "open" fixed the problem. Hopefully this post will help someone else with the same problem.