If you started reading this post expecting the answer then sorry this is the wrong place. Since the day Google Talk was launched, this is the feature which i missed the most (there were hardly any friends on Gtalk then but i convinced some of my close friends to install it). After its integration with Orkut lots of my friends are on Gtalk but the irony is that I cant chat with a group.
As a matter of fact, you can use spreadsheets in Google Docs to chat will all the collaborators, so there is a workaround but its not so elegant. I hope they come out with this feature soon.