When I first saw this at The H Open, in particular that it would use SIP, I was curious as to how it would difference from the modest variety of SIP clients already available. My experiences with SIP haven’t been great but I attribute that to other people’s choices of proprietary services rather than the clients or the protocols. Regardless, to my knowledge SIP usually does require that someone, somewhere has a server set up. To compete effectively with Skype either the server would have to be blindingly easy for non-sysadmins to setup or disappear altogether.
GNU Free Call sort of combines the two, building on the existing GNU SIP Witch project.
Instead of using any central provider or server, this enables multiple endpoints to find each other and connect. The announcement claims that acting as a self organising meshed calling network, without any central control, the system will continue to operate even if major parts of the network have been isolated. The project is being coordinated by Haakon Eriksen (firstname.lastname@example.org) and architected by David Sugar (email@example.com).
With the recent end-of-life for Gizmo5, I am very keenly interested to see this project succeed.