starting point: a working grub multi-boot installation, including a "production" back-level ubuntu system and a separate boot partition.

recovery starting state: successful standalone bootable partition with grub and a new ubuntu system.

recovery to multiboot with new and old systems active:

1. boot the only system avaialble on the grub menu - the new ubuntu.

2. mount the separate boot partition as /mnt.
     - copy the new kernel-related files from /boot to /mnt
     - merge the /boot/grub/menu.lst lines for the new system into /mnt/grub/menu.lst. don't forget to add/customise the automagic kopt line! don't forget to setup the correct root statements (intrepid syntax is not valid with hardy grub).
     - unmount /mnt

3. sudo vol_id -u /dev/sdaX (to find the UUID of your boot partition).

4. edit /etc/fstab on the new system. clone a new entry to mount /boot on the UUID from step 3.

5. mount your original boot partition as /boot

6. sudo update-initramfs -c -k 2.6.27-7-generic (or whatever you have as current kernel)

7. sudo grub
   grub> root (hdX,Y) e.g. /dev/sda7 is (hd0,6)
   grub> setup (hd0)

8. shutdown and reboot - you should see the original grub menu.lst, but including your new system.