You shouldn't log in as root under MacOSX. Make an admin user for yourself instead. Admin users are allowed to issue commands as root using sudo, and GUI programs have a special API for doing stuff as root (with an admin user's permission). This is a totally excellent security feature, since it lets you know when any application is trying to do something as root. So, no 3v1l trojan is going to replace /bin/ls or put your ethernet card in promiscuous mode without giving you some warning. (Compare to MS Windows, where toy pretty much have to be an Administrator all the time, or other unix-like systems, which are great on the command line, but break down whenever you start X.)

My advice is to try the .AppleSetupDone hack, or boot from the CD if that doesn't work. Do not enable root login, it's just another account that you keep track of, and you'll be happier without it.

BTW, to start in single user mode, hold down command (it's the one with the apple on it) and s while the system boots.