J. DeAngelo - AG6QO

How to change the Pi's root password -
 
If you are concerned about the security of your network-accessible Pi, you probably want to change the root password
from the distribution default. (I am assuming you've already changed the distribution user password (Pi account)).

Since the Pi's intended for easy use, the basic docs don't make it obvious how to do this.
Here's how to do it:

To change root password:
from pi (user) login
$sudo su ; to setuser to root
root@nodename $ passwd ; to set new root password

Now you can su any time you like. Just be careful!