[oops, I accidentally sent my reply to the wrong list, retrying]

> 
> The last time that I saw these symptoms, it turned out that used
> the serial boot blocks and was looking at the monitor...
> 

It should still print something like "NetBSD/i386 ffs primary bootstrap"
on the VGA console before switching (mine does, at least).

This problem can arise when BIOS vs. MBR vs. NetBSD have different
ideas on C/H/S translation. Try this:

Boot the cdrom with hd0a:netbsd as described in another mail.
# fdisk wd0a
Note the CHS numbers. Go back to BIOS and make sure it matches.
If it's 255 heads and 63 sectors you'll want to use "LBA" translation.
Boot cdrom again and do "installboot /dev/rwd0a /usr/mdec/bootxx_ffsv1".

Another thing to check while you're at it, does your "a" partition start
at sector 63 in the disklabel?

--
tnn