In Lion - how do you set up a root account?

The root account is enabled with Directory Utility, which is in /System/Library/CoreServices/. Unlock it using your admin password (using the padlock in the bottom left corner of the window), then go up to the Edit menu and choose Enable Root User. You'll be prompted to create a root password, which I strongly recommend be something non-trivial, and that's it. When you log out, you'll see a new "Other…" entry in the login window, where you can enter a username and password manually in order to log in as root.

EDIT: Be sure that you're aware of the risks involved in running as root, and never use it as your primary account.

jW