OK, when searching for this one the first hit is currently the one you're looking for. So let's just place a short copy here. The story so far: when converting your rootdisk to a RAID-1 as described by the really amazing guide, one might encounter, at least on netbsd-4.0.1/sparc64:

ok boot disk 
Boot device: /pci@1f,4000/scsi@3/disk@0,0  File and args:
NetBSD IEEE 1275 Bootblock
.Inode not directory
ok

...and then it stops. The solution is to install the bootblock again, but with a fixed version of /usr/mdec/bootblk:

# ftp ftp://ftp.netbsd.org
# get /pub/NetBSD-daily/netbsd-5/200812250000Z/sparc64/binary/sets/base.tgz
[...]
# gzip -dc base.tgz | tar -xf - ./usr/mdec/bootblk
# mv  usr/mdec/bootblk /usr/mdec/bootblk-5
# /usr/sbin/installboot -v /dev/rsd0a /usr/mdec/bootblk-5

Booting with "boot disk" will succeed, although this particular error message did not go away on our Ultra-60.
Update: PR 40306 has been opened to track this.