There's a new Dock modification app that recently came out for Mtn. Lion.
DockMod allows you to create an unlimited number of themes, with each theme each able to customize the look of the dock for an unlimited number of spaces. You can customize the top “floor” area of the dock, the front “edge” area, and the frontline, each as a different gradient capable of containing an unlimited number of colors. The opacity of each gradient is customizable, as well as that of the dock icon reflections and the icons themselves. An image can also be set in lieu of a gradient, or you can overlay the image on the gradient with adjustable opacity. The image will automatically adjust to fit the 3D perspective look of the dock. It’s so much fun, you can literally spend hours designing your favorite theme.
http://spyresoft.com
Used to use Leopard Docks and Dock Library but they no longer work in ML.