I'd like to set up a digital SLR camera, and have computer control of
the options and shutter release. I need to set ISO, Aperture,
Shutter-time, and Focus-distance (at least).

I'll accept solutions that come as a C/C++ programmer SDK for Visual
Studio on a Windows platform. However, even better would be command
line tool which I could just script from a shell.

(A sufficiently controllable GUI software application might be ok, but
I'm going to take thousands of similar images so it needs to have a
way for me to script which settings/shots I want to take. Most GUI
apps aren't designed this way. They assume you'll be clicking a button
for every shot.)

An acceptable answer will be of the form:

Canon: Has an SDK for their DSLRs. However doesn't provide focus distance.
 (some web link)
Olympus: Has SDK...blah blah
Nikon: Does not have SDK.. blah blah..
Fujifilm: Reportedly has SDK, but I can't find a link
...

Incidentally, I don't yet know about the other manufacturers. I do
already have the Canon SDK and as far as I can tell it does not let me
set the focus distance. The Nikon, Olympus data above is purely made
up. Its what I want to know.

I expect you can find this information partially from the
manufacturers and partially by observing what software already exists
to control different cameras. For instance since I didn't find any
software to allow a Canon to set the focus distance I presume my
understanding of the SDK is correct. You'll also have to figure out
who actually sells DSLR camera. This should be straightforwand using
dpreview.com or some similar.