Bit of a history here, but SIP had been around for a couple of years when Google Talk was released. I spoke to Chris Sacca, who was part of the Google Talk tea

Bit of a history here, but SIP had been around for a couple of years when Google Talk was released. I spoke to Chris Sacca, who was part of the Google Talk team (if I'm not mistaken) at Startup School a couple of years ago, and he told me that the 'SIP gateways' would be coming online shortly. This must have been in 06/07'. If memory serves well, the reasons why Google didn't go with SIP at the time had to do with federation and trust between different SIP domains. So it was mostly security and anti-SPAM related. They were meaning to do the right thing - i.e. being open - but found that SIP could not serve their needs.