I went through the Google Talk Developer pages. They say that they will support federation with other XMPP networks. XMPP is an open protocol, similar to email where user IDs have a username and domain. If you install an open source XMPP server – it’s also called “Jabber” – you could actually talk to other XMPP servers. Google closed that by choice, with their reasons explained on their page.
In the FAQ:
I am a communications service provider and want to federate with the Google Talk service. How do I proceed?
Please contact us at firstname.lastname@example.org
This is significant since it opens up connectivity to anyone who can operate an XMPP server. This includes telcos. Our local SMS carriers could operate a gateway between XMPP and SMS. While they are already doing this with Yahoo Messenger (“I’m on SMS”), the easy availability of XMPP tools could make Google Talk integration more attractive. Of course we don’t know about the commercial terms, if any.
The Google Talk page says they are going to integrate with SIP, a common presence, messaging and VoIP protocol. Our telcos are moving towards that. Smart has funded some mobile development companies and universities to develop SIP apps. Google did the same, funding XMPP development with “The Summer of Code” program.