We use an SD card for running some tests on plugs from time to time - I got fed up of logging in as root every time:
* Install mingetty
apt-get install mingetty
* Amend
/etc/event.d/ttyS0#exec /sbin/getty 115200 ttyS0
exec /sbin/mingetty --autologin root --noclear ttyS0
Now I see:
ubuntu login: root (automatic login)
See
http://blogs.koolwal.net/2009/04/30/autologin-linux-console-mode/