Wrong. We're talking iPhone Mac OS X here, not Mac OS X for Macs ... The root user is used standard, mobile is not used. When you turn your iPhone on, you are logged in as root. The root password is dottie. To change it, install MobileTerminal and BSD Subsystem, then execute this in the terminal:
passwd root
Then it will ask you to type the new password twice, and you'll have changed the password.