Note : this is an updated and (somewhat) tightened version of this hint at macosxhints for my personal reference. If it helps someone, great.
At work my computer is blocked from the outside world by a proxy. Here are the steps required to make iChat (Leopard) able to connect to Google Talk anyway.
1. Download the last version of proxytunnel. Version 1.8.0 compiles out of the box on Leopard, as long as you have the developer tools installed. Copy the proxytunnel binary somewhere nice, like /usr/local/bin.
2. In a terminal window, run
sudo /usr/local/bin/proxytunnel -a 5223 -p cache.bofbof.fr:8080 -d talk.google.com:5223&
(of course, replace cache.bofbof.fr by the actual URL or IP address of your proxy server)
3. In iChat Leopard, configure your google chat account as follows : server = localhost, port = 5223, Use SSL = yes, everything else ticked off.
After doing this, iChat should connect to your Google Talk account, and away you chat. If you are on a laptop, you’ll have to revert the iChat server setting to the regular talk.google.com when you leave the proxy jail, and change it back every time you come back to work (I guess proxytunnel can be left running without problems).
The original hint suggests a setup which takes care of doing all the heavy lifting through a customized /etc/hosts and Applescripts — they works pretty well as-is, although 1) the proxytunnel syntax will have to be updated to look like what’s above when using 1.8.0 and 2) there’s no need to call “nslookupd –flushcache” on Leopard AFAICS.
After all these efforts, 2 nice bonuses to reach chat nirvana : 1) Chax will give you a nice unified contact list and Growl notifications in iChat, 2) How to chat with MSN, Yahoo and other contacts through Google Talk.