OSXFAQ – Technical News and Support for Mac OS X:
Ever wanted to gain access to the UNIX command line without having to login into your user account? Sure you could start single user mode at startup, however, you are limited to what commands you can do. Luckily, Mac OS X makes it easy for you to login into the UNIX command line directly from the Login Window.
This was in earlier releases by default: you could log in as “>console” and do commandline stuff without the UI. I can’t recall if you needed to authenticate to run restricted commands and haven’t tested it in X.3.
But I will add this just in case I ever need it.