AutoCAD 2006 will include a technology preview of 3D DWF publishing as an option when you are installing AutoCAD 2006.
3D DWF files can be viewed in the free Autodesk DWF Viewer.
3D DWF publishing was included in Autodesk Inventor 9. Both Architectural Desktop 2005 and Autodesk Building Systems 2005 have an extension available to Autodesk subscription customers. There will be more announcements of 3D DWF publishing support in other products in the near future.
A link to a quick video (without audio) showing the creation of a 3D DWF from within the upcoming AutoCAD 2006. The video does not do the results justice due the video compression with limited color and resolution.
Using the AutoCAD sample drawing: Welding Fixture Model.dwg
The resulting 3D DWF is only 469KB