Jason,
I believe the issue might be in the way that you saved the file to .dwg
Did you create a new .dwg file in Inventor and create a base view of the 3D part or did you just save a copy of the 3D part to a .dwg file?
I know that I have imported .dwg files from Inventor 09 (created using the first method I described) to AutoCAD 09 without any issues. I will see if I can test this on AutoCAD 2010 but I will probably not be able to until sometime next week.
Best of luck,
- John
http://www.jdcncsolutions.com/