In a post yesterday about Google Talk being added to iChat 4.0, a reader named Mark pointed out that you can actually already do this in current versions of iChat via Jabber.
Simply select Jabber Account when you are setting up the account.
Enter talk.google.com as the server, and your gmail address as the username.
But keep in mind that the actual “Talk” portion of Google Talk does not work this way. It is text only.
Hopefully the new release of iChat will include full Google Talk support, with audio.