A very long time ago , Google commented to supporting SIP with Gtalk. Well with three years running, there is no SIP interoperability with Gtalk and Intenet voice is still a mess. What’s worse, is that few people seem to be caring.
SIP is used extensively, if not almost exclusively, through the telecom world for IP voice. Also it has the ability to be completely distributed like email. Why has it not been widely adopted for Internet telephony?