There is an article on VGLeaks about the Orbis OS. It's light on details but it has some photos of a dev kit booting. The screenshots look very easily faked to me but who knows.
FreeBSD would be an awesome choice. So many people can work with it well already and so many of the tools would be in place and ready to go. It's also a very solid foundation to start on as far as security is concerned.
It would explain why there is already so much middleware working on the PS4. It's great. It's an excellent way to kickstart high-quality games. It's a familiar hardware architecture running a familiar and mature OS.