Au contraire or somesuch 
It has always worked for me in Ubuntu, although I now run Mint (derivative of Ubuntu). In fact, you can give root a password and log in as su:
lou@dell690 ~ $ sudo passwd root
[sudo] password for lou:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
lou@dell690 ~ $ su
Password:
dell690 lou #
Lou