/boot on RAID1: "Can't open /boot"

I've tried to follow the instructions in "Chapter 15. NetBSD RAIDframe" 
for booting NetBSD from RAID1.

My configuration is NetBSD 3.1 over 4 250 GB SATA drives configured with:
  RAID1 root partition (mirroring across all 4 disks)
  RAID5 large partition (spanning all 4 disks)

The error I'm getting when trying to boot the system is:
Boot failed (errno  2): Can't open /boot

I thought the main tricks were:
 * make sure there's a partition visible with fdisk and marked active
 * make sure / is only RAID1 (since RAID5 not supported for root 
filesystem at boot time)
 * make sure / starts at the beginning of the RAID partition
 * make sure RAID is configured to set "root" and "autoconfig" (-A root)
 * run installboot for all bootable disks
 * based on the error message, I've double-checked the "/boot" file and 
ensured it is the same as in /usr/mdec

I can boot off a CD and mount the RAID partitions so I know RAID is 
working and the filesystems are ok.

Is there something I'm missing or anything else I should try?  Are there 
any peculiarities with RAID1 mirroring across all 4 disks instead of just 2?

Thanks,
Mike