I have a problem with iChat and my work around is to turn my computer firewall off (I have tried opening the ports that I thought were needed but this did not help). I connect to the internet using an airport base station. How exposed am I?
You should read the article bob posted and learn how to open those ports on a firewall. I go through two firewalls with no problem when I opened those ports. Apple's iChat work fine.
Going straight open on the net today is not the smartest thing. Take a look at the port scans on your firewall logs and you will see what I am talking about.