3d - Getting from a DEM to points that could be used for a TIN

I have a DEM, and would like to create a set of points that I can use to make a mesh for a 3d modeling program, such as Maya, Blender, or SketchUp. If I were to make a TIN from a DEM in ArcGIS, ArcGIS would create a greater density of points in areas where the slope changes.

How should I perform a similar process using open source tools? Do you know what algorithm is used for sampling points in a DEM to create a TIN?

I'm looking for a solution that would produce less points than the number of pixels, and in locations where the DEM is more flat, produces less points. The best solution for me would use python or command line tools and result in either a mesh-like geometry or points that could easily be triangulated to create a mesh.

Thanks.

[EDIT: would classifying areas by some combination of slope and aspect be a first step? maybe then I could put points along the borders of these areas?]