Comment 1 for bug 486361

I think I may have found the solution (or, at least, a workaround). I replaced all instances of device names of the form /dev/md_dX with corresponding names of the form /dev/mdX (e.g., /dev/md_d1 -> /dev/md1). This involved editing /boot/grub/menu.lst, /etc/mdadm/mdadm.conf, and /etc/fstab, and then running "update-initramfs -k all -u" to rebuild the initramfs'es. When I rebooted, the system came up on its own.

There may still be a bug / misfeature in mountall, however: Why did mountall hang when the root was on /dev/md_d1, but it worked fine when the root was on the (supposedly equivalent) /dev/md1 (and similarly for other filesystems on other RAID1 mirrors)?