>Is there any documentation on /sbin/bim?
The "help" command may be of help.

>In particular, what is expected for a "boot partition"?
You only need a boot partition if you are using the autoboot monitor.

Here is my usual disk layout:

  a - root
  b - swap
  c - raw (complete disk)   bim uses this device.
  d - boot partition 
  e-h other file systems.

The disklabel takes the first 1k of the disk.  The boot partition can live
anywhere on the disk.

bim (Boot Image Manager) adds the boot images information after the disklabel
in the first 1k of the disk (You need to use "disklabel -W sdX" to make
the disklabel writable.) and manages the images in the boot partition.
For use with NetBSD this is most often just the job of adding /boot
to the boot partition and marking it the default image.  From there, 
/boot can read kernels from the root file system! 

If you ran the autoboot monitor with minix, bim is just a highly hacked
version of hdsetup.  It has all the disklabel stuff removed because 
NetBSD has disklabel!

By the way, I think someone was working on trying to get NetBSD to
support more than 8 partitions per disk.

Hope this help.

--Phil

p.s. Yes there should be a bim.8.

--Phil