AAAAAARRGGGHHHH!!!!

"Pilot Error".  DXF does indeed import as 3D.  I made the mistake of exporting my TIN out as 2D only - so it goes when yr system is set up for making 2D plans. 

I'll report on the full AutoCAD2007=>Hexagon=>VUE workflow in a few days - after I get a chance to play with the conversions and test out the results.