If you want to open and edit DWG drawings produced from Inventor 2008 in AutoCAD 2007, 2006, 2004, 2002 and 2000/2000i (on a PC without Inventor), you will need the free Autodesk DWG TrueConnect Object Enabler (the required DBX is already included in the AutoCAD 2008 family). Without this enabler, some objects are represented by proxies only and you want be able to work with the Inventor view blocks

http://www.cadforum.cz/cadforum_en/qaID.asp?tip=5582