For the sake of posterity, the following commands will allow you to login to an otherwise unbootable Linux install and run commands from the install on the hard drive:
sudo mount /dev/sda6 /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
sudo chroot /mnt /bin/bash
With thanks to this part of the Ubuntu wiki.
For those are are interested, I managed to break my 64 bit install of Karmic 10.04 on my brand new Asus laptop by activating the proprietary (Nvidia) drivers when the install offered. After Usplash I see now is a black screen, however the system is working. Installing the version from Nvidia’s website did not help. Deleting xorg.conf (and forcing the system to use the fall-back VESA driver) worked.