I have been having problems with iChat for many months now. Not responding, or the other person not responding, and all of that nonsense when trying to do an audio or video chat. Sometimes the chats would work, most of the time they didn't. My setup is like this: A phone line to a DLink 302g modem to an AirPort Extreme base station, and finally, out to my PowerBook running 10.4.6.
The latest fix that I have tried, which seems to have worked so far, is to put the modem into "bridged" mode. I then connect to the ISP via 'Connect via PPPoE' under the Internet tab of the Airport Admin application -- add your ISP username and password in the appropriate boxes.
Hope this helps! I have been through the previous 212 iChat hints, and I don't think I came across a similar hint.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060513192721450