Have you compiled the kernel yourself? Debian kernels have the raid components compiled as modules, not compiled in.
If the raid system is compiled into modules, you need to state your RAID partitions in /etc/mdadm/mdadm.conf and rebuild your initramfs. If they are compiled directly in then the RAID partitions should be automatically detected, but the partition type needs to be set to FD (Linux RAID autodetect)
# Please refer to mdadm.conf(5) for information about this file.
ARRAY /dev/md3 level=raid10 num-devices=4 UUID=ffffffff:ffffffff:ffffffff:ffffffff
ARRAY /dev/md1 level=raid1 num-devices=2 UUID=ffffffff:ffffffff:ffffffff:ffffffff
ARRAY /dev/md0 level=raid1 num-devices=2 UUID=ffffffff:ffffffff:ffffffff:ffffffff
# This file was auto-generated on Wed, 22 Apr 2009 18:03:38 +0100
# by mkconf $Id$