Google just announced open federation for Google talk.
What it means:
""Open federation" is technical jargon for when people on different services can talk to each other. For example, email is a federated system. You might have a .edu address and I have a Gmail address, but you and I can still exchange email. The same for the phone: there's nothing that prevents Cingular users from talking to Sprint users.
Unfortunately, this is not the case with many IM and Internet voice calling services today. You can only talk to people on the particular service you have an account on (so you need an account on every service to talk to everybody, which is pretty cumbersome). With open federation, you get to choose your service provider and you can talk to people on any other federated service (and vice versa).
In addition to the Google Talk service, many other companies, universities, and corporations support open federation today. This means you can now talk to millions of users around the world all with a single account on the service provider of your choice.
More detail at http://googleblog.blogspot.com/2006/01/open-federation-for-google-talk.html