I wonder why they don't just release their existing AWT/Swing adaptations?
Either they are afraid of people using the Mac OS style on Windows, or that by open sourcing the style, they loose their copyright to it, or the code is such as mess that they'd rather not release it into the open.
How many years-old projects with deep hooks into a proprietary OS, written with no intention of ever being open sourced, would or could be opened on any reasonable timeframe? Any precedents spring to mind?
The old backend was written with quartz, and hopelessly integrated into the OSX internals. It's part of the reason why Apple had to ship their own java.
The new backend will only depend on OpenGL, which is much better for anyone who isn't looking to lock you into their format. Which, oddly enough, appears to be Apple's goal with it's handling of java.
Either they are afraid of people using the Mac OS style on Windows, or that by open sourcing the style, they loose their copyright to it, or the code is such as mess that they'd rather not release it into the open.