It's been exposed that the operating system powering the PlayStation 4 is Orbis OS, which is a Sony spin of FreeBSD 9.0. It's not a huge surprise FreeBSD is being used over Linux, in part due to the more liberal licensing. The PlayStation 4 is x86_64 based now rather than Cell-based, which makes it easier to use FreeBSD.
It's a bit of a surprise to see FreeBSD being used though when at the heart of the Sony PS4 is an AMD eight-core APU with Radeon graphics. There isn't a Catalyst driver available for any BSD operating system right now and the open-source driver ported from Linux with Radeon KMS and Gallium3D is still in a primitive state. Thus, Sony in cooperation with AMD must have developed their own graphics driver for the PlayStation 4 with FreeBSD or at least ported the Catalyst code-base to BSD.
The information of FreeBSD 9.0 powering the PlayStation 4 was shared at VGLeaks.com and based upon a second-gen development kit for the forthcoming console.