Thanks again Jon, I'm writing just to confirm this was indeed the sole cause.
Remaking initrd with Software RAID (using mdadm) already configured and
running, was the one step we had missed.  That was why our server was not
loading RAID at boot time through the kernel's autodetect mechanism.

For others who happen upon this mailing list thread via keyword search, the
important line in the kernel postinstall script is (variable names might not
be accurate):

mkinitrd -f ${initrdfile} ${version}

Where e.g.
	$initrdfile=/boot/initrd-2.4.21-27.0.2.ELsmp.img
	$version=2.4.21-27.0.2.ELsmp

Note: if testing and unsure, point $initrdfile to something else initially
and add a stanza in grub.conf to use it.

Dan W.

> The code to perform raid autodetection is added to the initrd *iff* raid
> sets are detected when mkinitrd is run (ie at kernel install/upgrade
> time).  Similarly the raid (and relevant scsi etc) modules are only copied
> if they are in use at the time.