The short version: Google now has an instant messenger. They have only released a Windows client. However it uses the Jabber protocol, more properly called XMPP and therefore any compatible client can connect (Tiger's iChat, Adiun, Gaim etc.). Your account name is the same as your Google mail account. If your have a Google Mail Account your already signed up, you just have to login with your client.
The most interesting part of all of this is the Federation of Service for the Voice over IP space. Google says: "We plan to partner with other willing service providers to enable federation of our services. This means that a user on one service can communicate with users on another service without needing to sign up for, or sign in with, each service."
It would be a very interesting world if Skype, Gizmo, iChat, etc. could all talk to each other.
If your interested in more Arstechnica has a nice write up about it:
"Google Talk does, in fact, use Jabber, and we're having success here in the Labs getting on to GT with GAIM, Trillian, iChat, and even Adium. Indeed, curiously, some of us up in the Orbiting HQ can connect to the GT network using GAIM, but not Google's own client. To make matters worse, Google only provides a Windows client, but as you can see there are other options for the moment. Mac OS X and Linux support are planned for the future. In the meantime, any client with XMPP specs should be golden." Behold Google Talk! A Mini-Review
(Submitted by Noel Thu Aug 25, 2005 )