I suggest you revise the thread name - it is not proximeter,
is photogrammetry

What we need is a photogrammetry application
and it would be highly useful measurement tool.
Unfortunately such a software would be somewhat difficult
to code from scratch.
On the other hand it might be best to start from scratch
rather try to cram code made for static desktops into our
beautiful little n900s which already have an excellent camera
ready to do this on-the-fly.

Here is a nice look at how it is done

One possibility but would probably need a lot of work: stereo

Another more complicated idea is an OpenStreetMap work,
but it looks rather involved to get set up and running.

But insight3d looks like the real deal (GPL3)
assuming it can be compiled on the n900

hope this helps