Bug#589836: I can confirm this problem

[Thomas Lange]

I have the save problem. No initrd is generated when I set up a clean
chroot and also install mdadm into it. Without mdadm, initramfs-tools
will generate an initrd.

I have the same error massage.
> sed: can't read /etc/mdadm/mdadm.conf: No such file or directory
> E: /usr/share/initramfs-tools/hooks/mdadm failed with return 2.

For FAI, it's very important to get this fixed. If you need more help
for debugging, I may help finding or testing a fix for that.
-- 
best regards Thomas