A footnote to this. I did mdadm --manage /dev/md0 --add /dev/sda1 and mdadm --manage /dev/md0 --add /dev/sdb1, and now the output from update grub is different:
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
Generating grub.cfg ...
Found linux image: /boot/vmlinuz-3.2.0-4-amd64
Found initrd image: /boot/initrd.img-3.2.0-4-amd64
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
/usr/sbin/grub-probe: error: disk missing.
Found linux image: /boot/vmlinuz-2.6.32-5-amd64
Found initrd image: /boot/initrd.img-2.6.32-5-amd64
Found linux image: /boot/vmlinuz-2.6.32-trunk-amd64
Found initrd image: /boot/initrd.img-2.6.32-trunk-amd64
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
Found Debian GNU/Linux (7.0) on /dev/sda1
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
Found Debian GNU/Linux (7.2) on /dev/sdb1
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
Found Debian GNU/Linux (7.4) on /dev/sdc1
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
Found Debian GNU/Linux (7.4) on /dev/sdd1
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
error: superfluous RAID member (2 found).
error: superfluous RAID member (2 found).
error: disk missing.
error: disk missing.
done