Widely publicized already, I felt compelled to add to the chorus when I put this change in. Below are terminal commands you can use to switch back and forth between the new 3-D and 2-D dock configurations.

To enable the 2D Dock, open up a terminal window and enter the following two commands

defaults write com.apple.dock no-glass -boolean YES; killall Dock

To enable the 3D dock, open up a terminal window and enter the following two commands

defaults write com.apple.dock no-glass -boolean NO; killall Dock

Thanks to Proton over at MacOSXHints.com for originally posting this tip!