So what exactly is/was PSSG? And yes it is a multiplatform engine.
Gotta be careful here, I can't remember exactly how much Sony have said in public - but they presented at Develop in the UK, so I'm sure they've described it in reasonable detail.
PSSG is a "high level" graphics engine - it's got scene management, animation, that kind of thing. It's also cross-platform, which pretty much means it runs on PC as well as PS3, because like I said I don't think Sony have a 360 devkit lying around
There's not much "game" stuff in it - it really just throws graphics around. But it seems pretty capable - you can make complex animated scenes in your 3D package, export them (assuming you can get Collada to work, as it uses that format), and it'll load and render them for you.
There's also some tools and stuff, and it's all pretty open for extending (IIRC it's all source-code so that you can port it or change it, but there are APIs for extending it rather than just having to hack it).
Like I say, they did a presentation at Develop on writing a simple game with it - not sure if that's been made public or not though.