(The person who gave me the board said the machine was owned by a local
    electronics college at the time he had access to use it, so they can't
    be super-expensive, but I'm sure they're not cheap...)

There are two major companies I know of that make CNC PCB routers.  T-tech
(www.t-tech.com)and LPKF (www.lpkf.com) I've seen others at trade shows, but
they haven't been around as long, and don't have the exposure or reputation.
Entry-level models run about $10k, plus another couple $k for necessary
vacuum system, tooling, and so on.  Used models are available for cheaper,
sometimes.  There's an LPKF machine on eBay right now at an current price
of $4.5k (and it's one of the larger models.)  And you need a computer to
drive it (uses a serial port.)

I've been using an LPKF, bought from eBay, for maybe 6 months now, and have
run off perhaps a dozen small boards.  (very small boards.  In fact, that's
my motivation for getting the machine.  Start designing 4x6 inch boards and
those prototype shops start to look pretty attractive at ~$50 each.  Design
a 0.75x2 inch board that might not work, and you're still looking at $50 for
way too many of them from conventional shops...)

The way these things are built leads me to believe that it would be
relatively difficult to convert a standard plotter or CNC machine to do
similar tasks.  They're SOLID, and the motors needed to move a router bit
removing metal at substantial speed and 0.01mm accuracy are pretty hefty.

Software wise, they look like HPGL plotters.

Eagle (and presumably other CAD packages) will generate "outline" data for
tracks (also in HPGL format, if you want), so in theory you don't need much
additional software.  In reality, both plotters come with substantial
software that adds a lot of capability (multiple layers, milling, board
placement and multiplication, "rubout" functionality to remove extra copper,
tool and motor management, and more.)  They charge a lot for the software,
and it's probably worth it (although the LPKF software can be agonizingly
slow, compared to the Eagle outline functions, for instance.)

There's a reason that you usually only see the bare minimum of copper routed
away.  The tooling (carbide bits) that these things use doesn't last that
long, and standard pricing is quite high (~$17 milling bits.  Each.)  There
aren't a lot of second sources, either (unlike drill bits.)  Grr.

There's a registration system that allows double-side board to be routed,
but holes are not plated through, of course, which I'm finding has nasty
effects on the way you might lay out a board.  Single-sided circuit boards
work better, but single sided material seems to be relatively hard to find,
and if you have double-sided material you have to route both sides even if
your board is only single sided.

They're very noisy.  Especially the vacuum cleaner.

I don't think I wasted my money, but it's also not quite as wonderful as I
thought it might be.  By the time I use up the tooling that came with my
machine, I may be ready to re-think whether it makes more sense to use the
board-houses.  (Of course, but then I also expect to have a bit more faith
that my PCBs will work...)

BillW

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