I’m a bit late on this, but I’m psyched to see that last week Google flipped the switch to allow all their Google Talk instant messenger accounts to talk to any other Jabber client out there. I’ve not verified it yet, but I think that included people with .Mac accounts using iChat, and BigBlueBall has a nice tutorial on how to use the federation to hook up your GTalk account directly to AIM, Yahoo!, MSN and ICQ using Jabber transport services.
This is the final step I’ve been waiting for before ditching my AIM account and going entirely to Jabber!