I have to say that I'm pretty jealous of talk.google.com - I really, really like Jabber. Having an open and extensible standard like XMPP as the base of your IM solution is pretty awesome. Ever since I wrote that Jabber client for J2ME YEARS ago, I've really liked the XMPP protocol. It's just so easy to use, even from something as lightweight as a J2ME 1.0 client. If you can open a socket, you can use Jabber - that's pretty powerful.
I don't know if you know how it works, but it's pretty wild. You don't send individual XML documents per message, instead you open up a socket and start writing one XML document keeping the socket open the entire time, as you need to send more messages, you keep adding XML stanzas to the document. You do this both on the up and down stream. To end the conversation, you simply end your document's root tag. Now think about this - if you're really just sending a never-ending XML document as the way to make a conversation, then extending this protocol is drop-dead simple. You just add another namespace and include new tags for that namespace in the document. *Poof* - extensible instant messaging and presence. They've got a ton of extension proposals already in the works, including sending forms, multi-user chat, and geolocation. I mean, it's very cool.
I'm not sure how scalable XMPP is, or why Yahoo! hasn't switched to it yet, but I'd love to see us put an XMPP gateway at Y! and start letting people access Yahoo! IM via Jabber as well as via our custom client. Our IM client is amazing (with integrated Music, Search, Webcam and Voice), but choice is always better - and then ISVs could start piggybacking on our stuff as well. You still need a Yahoo! ID, so it's still a win for us... I'm not sure where the decision is kept, but it'd be neat if this turn of events prompted us and MSN and AOL to open up a bit.
Now, let's see what the Goog launches in terms of a client and see what sort of innovation they've added.