As reported here, and experienced last week internally within Nova, there are issues with importing older 3D models from Mechanical Desktop into Inventor.

Link : “Yesterday, one of my collegues had problems importing his old 3D DWG model into Inventor 2009 on his new workstation. He was sure this or similar model went fine before.
The reason of the problem was simple. This DWG file originated in Mechanical Desktop. And for importing MDT models into Inventor you need Mechanical Desktop installed on the same PC. As he did not installed MDT with new Inventor 2009 (it is not a part of the installation), MDT files could not be imported. Simple installation of MDT has fixed the problem.
It is important Autodesk will keep Mechanical Desktop (even when it is no longer developed) available for future Inventor versions.”

The progressive phase out of MDT started with the release of Inventor 2009 as reported here. It will be interesting to see Autodesk’s solution to this as and when V2010 is released.