Generating TIN from DEM using MacOSX/Unix tools?

You have a few options:

1. PostGIS 2 has support for TIN and some other 3D objects.
2. GRASS has support via an add-on. See this how-to document.
3. QGIS has an Interpolation plugin (see here)
4. Roll-your-own by importing the GDAL libraries into a 3D modelling package such as Blender (I do it this way a lot because I want interactive 3D model rather than for analysis - but you can always export the result to DXF or Collada).

All these approaches should be possible on Linux and I think on Mac too.

EDIT: I've just remembered another two for you: Terragen (commercial and free versions) and LD3T. LD3T theoretically works on Linux but they recognize that there are issues. I've only ever run it under Windows. It's worth noting that the Terragen format is readable/writeable by Blender, Landserf (see SS_Rebelious' post) and LD3T.

Also for this purpose at your service:

1. SAGA GIS (has TIN module and a lot of tools for DEM analysis and manipulation)
2. LandSerf (java-based, concentrated on DEM and TIN manipulation)