The parameters caught by Firebug for a POST request that was generated when placing a call from Gmail:

count	3
ofs	41
req0__sc	c
req0_jid	c1712561387
req0_json	["jmistart","","c1712561387","Thu Nov 04 10:04:12 2010","a"]
req0_type	j
req1__sc	c
req1_jid	c1712561387
req1_json	["jc","","c1712561387",[["","57208","rtp","gNrtqsy8IWpSbE6e","ZBX31w0dgOdbjkfw","1","udp","0","local","0"]]]
req1_type	j
req2__sc	c
req2_jid	c1712561387
req2_json	["jc","","c1712561387",[["","57209","rtp","Yn/0kLMEYxKhZq9w","om4GTLNgDGXmfsJK","0.9","udp","0","stun","0"]]]
req2_type	j

The Gmail dialler also loads a flash plugin to handle the audio and video.

I placed the call from Firefox 3.6.12 which doesn’t support the new HTML 5 web sockets. It’s possible that in browsers that support HTML5, such as Chrome and Firefox 4, the Gmail dialler does use XMPP. That would mean that Google Voice would have two different code bases for the dialler. For most companies that wouldn’t be feasible but Google has more programmers than most companies so it’s entirely feasible.

I also just had a quick trawl through the Asterisk mailing list and Google Voice support in 1.8 still seems to be a bit hazy. The source code comments indicate a definite intent to support calls over XMPP, no mention of jingle. But when someone posted a question about how to acheive Google Voice calling with Asterisk 1.8 most of the answers describe various HTTP callback mechanisms, the same as what sipsorcery currently does. Some people do seem to be having success with originating Google Voice with Asterisk 1.8 and XMPP calls but despite following the same steps it didn’t work for me.