DWG is a proprietary and undocumented file format owned by Autodesk (which makes Inventor). Since they don't release the specifications, and can change them at will, it's difficult for an unrelated company to offer full compatibility with it. Here's a document from some folks who have tried that:
http://www.opendesign.com/files/gues....dwg_files.pdf If you don't like using STP, you could try IGS, or if you're exporting meshes, the STL format, which is generic and pretty universal (if not particularly compact).