Max OS X Tiger
"More than even the processor... more than even the hardware innovations that we bring to the market... the soul of the Mac is its operating system... and we're not standing still!" -Steve Jobs, closing his keynote at WWDC 2005

I wonder if that means Apple’s going to release Mac OS “leopard” for PCs also…

That would definitely be inline with the Mac Mini release. They admit they have been running secret projects for years. And all they really still need is a specific hardware abstraction layer as well as a few drivers… Definitely possible…