Many Linux distributions permit the use of the root user as a primary login, however, the dominant and popular Ubuntu has left some users in a conundrum. In order to perform tasks as a root user, this must be performed within a terminal window with the sudo command. However, here are a set of instructions that will allow the root user to login to an Ubuntu system.

• Press Alt-F2 to Run Application.
• Type gksu gnome-terminal in the Run Application window.
• Type passwd root to create a new password for root.
• Type sudo passwd -u root to enable to root.

Source(s)
http://www.liberiangeek.net/2010/08/unlock-root-account-ubuntu-10-04-lucid-lynx/