It more than likely is Firewall related. However, even in ideal conditions (IE, properly configured firewall/Apple router etc), iChat often still wont work as it should. Its incredibly touchy these days.
You'll find a multitude of similar reports on the Apple discussion forums. It usually doesnt work anymore for me either..
I'd still recommend you take a look in the Apple discussion forums however, as theres a couple of guys in there who are very helpful and even made a good website with great guides on how to fix such problems. Well, I say "Great".. They do their best to help, but in my case, its still borked.