Add to /etc/rc.local before the ‘exit 0’ line:

mkdir /var/run/screen
chmod 775 /var/run/screen
chgrp utmp /var/run/screen
su USER -c 'screen -m -d -S SCREENNAME bash -c "COMMAND"'

Replace USER with the user you want the command to run as. SCREENNAME with any name you like, and COMMAND with the actual command to run in the screen.

The first three lines were necessary to work around a bug in screen on Ubuntu.