It appears as though a private developer meeting for the next-generation Microsoft console has been leaked by a developer from Crytek, who was attending the "Durango Developers Summit."
Sean Tracy, who works at Crytek, tweeted "Enjoying the Durango developers summit in London. So far, great swag and interesting talks." The tweet has since been removed by Tracy.
This could confirm the existence of the "Xbox 720" once and for all, but also what the codename for the project is at the moment. Microsoft have been known to use code names in the past for their products, for example, Kinect was originally dubbed Project Natal.
We can expect to hear more about this at E3, especially as more and more developers are now talking openly about developing for the next generation of home consoles. The most recent of which was Bethesda, who're working on an unannounced title.