> New installation on an ABIT BP-6 dual Celeron MB, both slots with Intel
> Celeron 400s, upgraded to the lastest official BIOS (version RU) to get
> support for extended INT13.

Hopefully this works - since you WILL be using it later...
> Secondary/Master.
> 
> Two BIOS partitions (0 and 1), both id 169 (NetBSD). fdisk sees these and
> displays them just fine.

You won't be able to boot the second - unless you get all my
changes to the boot code...
> 
> Partition 0 is 10g, set active, and has a disklabel and NetBSD filesystems
> (a = /, b = 512M swap, c&d standard, no others)
> 
> Partition 1 has not yet been set up in any way, other than it's existance
> in the BIOS partition table, and has the other ~30g of the drive.
> 
> fdisk -i -c /usr/mdec/mbr_bootsel (configured to boot partition 0 after
> 10 seconds, has an named option for partition 1 as well)

Ok the presence of the the entry for partition 1 beyong 8Gb will
have made fdisk set the flag that causes the mbr_bootsel code
to use the int 13 entensions to do LBA reads.
(You can't clear this flag using the standard fdisk)

> Base install 1.5.1, upgraded to CVS branch netbsd-1-6, complete rebuild
> and install of kernel and userland according to "Tracking NetBSD-current".

Did the 1.5.1 version ever boot?
> 
> /usr/mdec/installboot /usr/mdec/biosboot.sym /dev/rwd0a
> 
> Upon reboot, the boot selector runs, expires the 10 second timer, and at
> that point prints a "3" on the next line, then hangs.

NOOS - ie the sector read didn't end 0x55aa.
 
> Booting from the install CD, interrupting the auto-boot, and entering
> "boot wd0a:/netbsd" works just fine, for both the old and new kernels

That only requires the filesystem to be present, not any of the
boot code.

I'd double check that the first sector of the netbsd partition
contains the boot code.

Note that the root filesystem (or rather the one passed to
installboot that contain /boot) must be at the start of the
bios partition.

With a few code tweaks you can get the mbr_bootsel code to
select a different boot partition and hence root partition...

	David

-- 
David Laight: david@l8s.co.uk