I'm not going to go over basics such as hidden files, how to use sudo, etc. You will need to start with a bit of knowledge to do this. Here are the general instructions for how to do it:
1. Locate and make backup copies of these 4 files located in /System/Library/CoreServices/Dock/Contents/Resources
2. Modify them in your favorite photo editor. I used Photoshop and painted them black then reduced the layer opacity to around 50%. You can experiment with colors, gradients, and even add textures. I went with flat black because I felt the curve in the original made the dock appear too busy.
3. Replace the originals in Dock.app (at the path above). Restart the dock (killall Dock).
Thanks to Marvin for giving me the idea to try this. If you know of any other ways to customize the dock or any other new Leopard interfaces please share.
By request, the PNG's I used for the Dock above can be downloaded here: http://rapidshare.com/files/66838383/Black_Dock_Mod.zip