Make sure these ports are accessable for ichat in the router setup.
Apple use the following ports for ichat that I have found
iChat AV Port 5060
ichat Port 80
ichatAgent Port 5190
iChat and Jabber messages Port 5222
iChat (local traffic), Bonjour Port 5297 & 5298
ichat/ \tMobileMe (Automatic sync notifications) Port 5223
iChat AV (Audio RTP, RTCP; Video RTP, RTCP) Port 16384-16403
These may not all be used or needed for me only Ports 80 & 5190 are used so far but I have seen others asked for when using Little Snitch.