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 Mecanical Desktop (even when it is no longer developed) available for future Inventor versions.