Preview at WWDC likely in June, followed by September launch.
Mac OS X Leopard 9A581's Dock Visual Tweaks [Updated]
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Apple has updated the Dock so that it has a 2-Dimensional appearance when it resides on the side of the screen. When residing on the bottom of the screen, the Dock retains the 3d shelf appearance.
Mac OS X 10.5 Leopard will officially launch on October 26th at 6 p.m. If you haven't already ordered online ($20 off), MacRumors readers are organizing meetups in their local regions.
Update: Apparently the same simple style can be used for the dock when it is on the bottom: screenshot