Use the 3DDWF command to create a 3D DWF file from AutoCAD. (EXPORT command also works). This example is in a DWFX format. Rain Barrel in 3D File viewers from Autodesk are found here.
I discovered the 3Ddwf file I created in AutoCAD 2011 got an error when using as an underlay in AutoCAD 2010. It appears 3dDWF files are not backward compatible for underlay purposes. I also tried using a DWFx format with the same result.