Just to clear things up a bit for anyone wanting to machine polygon (stl, obj, 3ds, etc.)meshes. My name is Jay Delbeke, and I do 3D design and CAD/CAM for Stargate SG1 and Atlantis. YOU CAN MACHINE STL'S. I've been doing it for years in foam, plastics and metal, at remarkable resolutions. Inevitably, all Nurbs and solid models are calculated by "highend" cam programs as polygons. The tolerances you choose dictate how small the calculated facets will be on curvature. As far as I caqn tell, there is only one real cam solution for stl/polygon models. Visual Mill is the CAM software you must have if you want to cut scans. No, you dont need to make closed solids to machine a shape. Even a one sided scan shell open on the bottom can be put into VM and toolpaths generated in moments. It's relatively verry cheap, and super easy to learn if you have a good understanding of CAD/CAM. You can build suport geometry right in the program, as well as regions and circles/arcs for hole machining etc.
The first problem you will incounter with high triangle scan meshes is manipulation and separation of geometry. For most people there is no easy solution because polygon modeling is a whole career field unto its self, but there are tools that are free that can do it. Look up Wings 3D, its a free polygon modeling program made by professional poly modelers. Easy to use.
You often have to cut areas off and lay them flat for machining then reasemble. I am including a picture of A full body I scaned, cut into 3 " slabs, and layed flat to be machined on a Shop Bot with a 4" gantry!!!! And it came out perfect. Now that's a feat, if you know anything about a Shop Bot.
In some cases, you may know how to do multiple opperation cuts. You can create a tool path of one side of an object, save it, flip it over on it's center and toolpath the back. You now have 2 toolpaths. Cut one side, flip the stock over, cut the other side and most of your model will be free of the stock. Using a CNC mill like a haas, with vices and clamps, this is easy. On a CNC router table its a bit more tricky, and you need to make registration marks or pegs in the stock and table bed to flip acurately. I routinely do 2-4 sided cuts on cheap CNC router tables that are aligned perfectly, but it's an art.
If there is lots of interest in these techniques, i may write a tutorial with pics.
Ayway, the point is there is more than one way to skin a cat. machining models (almost) right out of the scanner is possible and routinely done.