Looks like Apple's still implementing some of their security features. The file /etc/ttys (which appears identical to 10.0.4's version) labels ttyp0 through ttyq4 as network and not secure, which causes root to be unable to login (probably setup this way to keep root from logging in through telnet).
Is there any reason why you don't just 'sudo somecommand' or if you need a regular shell as root, 'sudo -s'?
If you must, you can edit /etc/ttys and tag any ttyp? or ttyq? entries as secure, then 'login root' should work...