I just confirmed that this bug is still present in today's d-i (daily built
from the SVN, r45447).
We booted the Xserve on the serial console, did the installation normally, and
in partman created the RAID partition on a mac partition table. This failed to
create the RAID flag, and thus partman-md failed to detect the disks, with the
error message shown in earlier reports.
Going into a shell, setting the flag by hand, and then going back to
partitioning and it works fine, so maybe this could be added to the erratas.
Notice again that there is absolutely nothing in reproducing this which needs
powermac hardware to test, and i am sure that i can even be reproduced in
quemu or vmware.
You just need to chose a mac partition table, and you will face this bug.