When I log in as a Guest user on Mountain Lion, i get a transparent dock with no background 'surface':
I can seem to find the settings that would make the dock in my own users' desktop look like this. Is it even possible for non-guest users?
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You could try a program called Mirage, which should make the dock transparent.
Mirage (as mentioned by ekaj) is a great solution, but it is no longer being updated and the developer's site has been down for a long time. I wrote an app called MacUtil that gets the job done, look here for a short video showing it in action.
MacUtil is a customization application for the Mac. It allow you to make your mac your own, with lots of customization options ranging from two dimensional docks to modifications of CoreOS files.