As I mentioned to James Seng in a comment I posted to his Google Talk post Google Talk isn't using SIP (yet). James confirmed my packet capture analysis of Google Talk with some sniffing of his own, which you can check out in his More About Google Talk blog post. He has some interesting discoveries, including Google appearing to install a STUN server on every Google Talk client to solve the VoIP NAT issues. James writes "In other words, like Skype, Google Talk turns every client into a possible server to help relay voice call between two users." Doesn't this sound eerily like Skype's super nodes that everyone gripes about?

James has echoes my sentiments when he writes, "Very smart thing to do technically speaking but let me go read the Google Talk UAT again. (Hmm...What happen to the 'Don't be Evil' plan?)"