stacked raid devices not autodetected?

I have several raid devices that I created with the following
commands:

    mdadm --create /dev/md0 --level=mirror --raid-devices=2 /dev/sd{b,h}1
    mdadm --create /dev/md1 --level=mirror --raid-devices=2 /dev/sd{c,i}1
    mdadm --create /dev/md2 --level=mirror --raid-devices=2 /dev/sd{d,j}1
    mdadm --create /dev/md3 --level=mirror --raid-devices=2 /dev/sd{e,k}1
    mdadm --create /dev/md4 --level=mirror --raid-devices=2 /dev/sd{f,l}1
    mdadm --create /dev/md5 --chunk=256 --level=stripe --raid-devices=5 /dev/md{0,1,2,3,4}

On boot, the kernel automatically detects and starts md0 through md4.
However, md5 is *not* autodetected.  Using the RAID_AUTORUN ioctl also
has no effect.  The only way I've found to reliably start md5 is to
have rc.sysinit run "mdadm -assemble --scan".

Is this a bug?  A design limitation?  Something else?

(I am currently using kernel-2.6.9-1.675_EL on the RHEL 4AS beta.  But
I've never been able to get autodetection of stacked raid devices to
work using any kernel.)

-- 
James Ralston, Information Technology
Software Engineering Institute
Carnegie Mellon University, Pittsburgh, PA, USA