The only way we found to get a triangular mesh with existing topology into ArcGIS was to use the `arcpy.LandXMLToTin_3d` function.  It's a bit crazy to have to write out a LandXML file just to get your TIN into Arc, but it works.  Here is a Python Toolbox for ArcGIS10.1 that accesses data on a triangular mesh from an ocean model, and brings it into ArcGIS as a TIN.
https://github.com/rsignell-usgs/dap2arc/blob/master/dap2tin.pyt
We've let ESRI know that a great enhancement would be to be able to instantiate a TIN directly using arcpy.