Quote - Just a few general facts about sub-d.  The Catmull-Clark sub-d subdivides the entire mesh, not just parts of it.  There are specialized sub-d types used by major CGI studio that can selectively subdivide a mesh but I don't see us getting them any time soon.  Sub-d adds polygons but it does not add detail, if anything it actually smooths out some details.  Take a cube for instance, add 1 level of sub-d and it clearly isn't a cube anymore, add enough levels of sub-d and begins to look more like a sphere.

Actually, as described in this thread:

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2855662&page=2

'Keep Edges' can retain the shape as you're subdividing.

Also Genesis deals more with subd cages. These are not really meshes but instructions of how the subd should handle the mesh.  Subdividing a regular mesh may eventually smooth it out without keeping edges and lose detail, however cages may bring in more detail, which you can see if you look at some morphs at no subdivision version adding detail in since the cage is providing points of where move the mesh as well. (which is probably the simplistic explanation)