Ubuntu 10.04.3 LTS has an apparent bug when running grub-install onto the MBR during the post-install phase. Neither grub2 nor grub legacy want to install. To get grub to install, you need to chroot into the installed environment (takes me back to my Gentoo days..) update, then retry installing legacy grub. To summarise:

Press CTRL-Alt-F2 to switch to PTS/2, then [Enter] to activate the console.

Mount virtual filesystems:

mount --bind /proc /target/proc
mount --bind /dev /target/dev</pre>

Chroot into the environment, update and install grub:

chroot /target
bash
apt-get update
apt-get -y dist-upgrade
apt-get install grub
CTRL-D
CTRL-D

Now switch back to the ncurses installer on PTS/1 with CTRL-Alt-F1 and re-run the grub install step, making sure to select legacy grub.

Apparently a fix has made it into 10.04.5..