To login to OS X 10.1 as root, first login as your normal Admin user. In the Login pane, click the Login Window tab, and then check the box that reads 'Show "Other User" in list for network users'. Logout after saving the change. When the login box reappears, you'll see "Other User" as an option. Click it, and you can enter "root" as the username followed by the root password.
In over a year of OS X usage, I have not yet had to login to the system as root to accomplish any tasks. But if you do need to (and there are some valid reasons), this is how it works in 10.1.

