An experiment into CoreGraphics and HITheme for drawing UI components.
I found out with CoreGraphics & HITheme you can draw interface components relatively fast without the overhead of creating Cocoa buttons and altering their bounds each frame, so the obvious thing to do was make a game with it!
Its also known to work with OS skins like the Amora Theme for shapeshifter, thanks to Robin for the screenshot.
You can download the game (universal binary) or source code here.