Of course Jeff Nolan was right, but the thing is, there are already Jabber/XMPP transports that work with AIM/ICQ, MSN, and Yahoo. I’ve used Jabber for years and they work just fine. You do need an account on all the other services to talk to your friends. But Google might add features in their client on top of the regular XML-based Jabber, so you can convince your friends to use their chat client, and then after a while they might very well switch to using Google if the voice (or video, or whatever) feature only works there.
The problem with Jabber is that it was never easy to use, and it was never very stable. To register my Jabber account on my friend Dan‘s server I had to telnet to a certain port at the server and type in XML code (now most clients support registrations). jabberd, the Jabber server, was sorta a hack, and seems like nobody cared enough to maintain it. The MSN and Yahoo transports were also not maintained. With Microsoft and Yahoo trying to block other clients from using their services every other day, they didn’t work half of the times. This was true until very recently, I’m not sure what happened, maybe Dan installed newer versions of the server and transports. Jabber has worked flawlessly for the last several months.
Also, if you domain name doesn’t resolve to your Jabber server, you’re pretty much screwed. And I wanted to have email@example.com instead of firstname.lastname@example.org. I had the vision that Jabber will eventually replace all the IMs and it would work more like emails, with distributed servers that handle IMs. The Jabber specs supports _jabber._tcp SRV DNS records, so in theory if I include a line in my DNS entry to indicate to where Jabber TCP traffic should go (sort of like the MX entries), it should work. But I don’t think it ever did, last time I tried was 3 years ago though, so this might not be true now.
Google, once again, took an existing technology, vastly improved it (I’m sure they rewrote most of jabberd), added features to it (voice), and rolled it out as a brand new service. Probably 90% of the Google Talk users had not heard of Jabber before. This is pretty cool.