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?
To make the dock transparent in Mountain Lion type the following two commands in a terminal:
defaults write com.apple.dock hide-mirror -bool yes
Mirage (as mentioned by ekaj) is a great solution, but it is no longer being updated and the developer's site has been down for a long time.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.