As explained in the (QT video) demo of the SUPER OS 10.5. It seems the new dock will have a 3D look:

It’s therefore notable that Leopard sports an entirely new Dock appearance, which now presents its icons as three dimensional objects sitting on a translucent glass shelf, rather than flat objects stuck in a translucent ribbon of icon flypaper. Dock icons now cast a reflection on the Dock (as do other windows on the screen, and even the background image), as well as casting a shadow behind them. The combination of the reflection and the shadow contribute to their dimensional appearance.

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