Can't reproduce with current version of grml-debootstrap:
* mdadm --create /dev/md0 ...
* grml-debootstrap
* use /dev/md0 as target
Works as expected, closing this bugreport therefore.
If SW-RAID isn't set up within grml-debootstrap installation of grub is
skipped, as "$SELECTED_PARTITIONS" is empty and GRUB/GROOT aren't set
accordingly.