I have found a tool in a couple different locations that helps figure out what your firewall is doing to your packets and whether it is actually interfering with your ability to use iChat A/V.
Tim Pritlove’s page on troubleshooting iChat A/V has some information, but several of his links are either broken or outdated.
I found that his
natcheck program, however, worked fine. It correctly diagnosed that my firewall was OK, and my friend’s firewall was not. The kiss of death is what you see related to consistent translation. Yes is good, No is bad. No way around it. Either you do consistent translation or you don’t. That will determine whether iChat A/V (and similar peer-to-peer technologies like Bit-Torrent) will work for you.