Sounds like Star Trek, but it’s not. As the post mentions, federated systems allow different services to talk to each other, like email and phones — you don’t have to belong to the same domain, for example, to be able to send email to each other.
The Google Talk service uses XMPP (Extensible Messaging and Presence Protocol), which allows clients using the same protocol connect to each other. Interestingly, home-brewed Chikka is mentioned as part of the open federation that Google Talk will be connecting with. Google Talk says,
We currently support open federation with any service provider that supports the industry standard XMPP protocol. This includes Earthlink, Gizmo Project, Tiscali, Netease, Chikka, MediaRing, and thousands of other ISPs, universities, corporations and individual users. (emphasis mine)
What this means for us mere mortals is that we will be able to communicate with other people on a different service as long as they talk the same language (XMPP), without signing up for that service: one account, one service, multiple connections.
I don’t use Google Talk that much (and I haven’t tried its voice capability), but it’s worth looking into. And for those who want to implement an XMPP-compliant IM service, there are plenty of resources (Jabber is one) out there. It’s open source, by the way.