Comment 6 for bug 712173

Adding the following to the kernel command line from the CD bootloader should do the trick (assuming you have an ethernet connection):

d-i preseed/early_command string chroot root apt-get -y install openssh-server

Alternatively, you can add break=bottom to the kernel command line, then:
(initramfs)
chroot root bash
mount -t proc proc /proc
mount -t devpts devpts /dev/pts
dhclient eth0
screen apt-get -y install openssh-server
exit
exit