AutoCAD MEP works in levels and so, expects Revit to export the building by levels. This is just extra work for architects. If the MEP guys had Revit, architectus can just burn the rvt project in a CD and call it a day. However, we had to export to AutoCAD and these are my notes to export to autocad each floor as a separate 3D dwg file.
*****update 20080731 - instead of using the elevation, one can use plan views too... as long as the view range in plan extends only to the level below.
Also as Scott commented below one can use IFC export and import it in AutoCAD (version 2008 and above...) ******
1. Duplicate an elevation and crop the boundary to display only the first floor. (the horizontal crop boundary aligns with project Levels.
2. Go to a 3d View. Rename it MEP-First Floor; Go to View Menu > Orient > and select the cropped elevation. Use the middle mouse button + shift key (or the view cube) to rotate the view.
3. Switch off unwanted elements for MEP. Create a view template from this view. This view template can be used if the project has multiple floors.
4. To setup other floors, go to the cropped elevation. Stretch the crop boundary to encompass the next upper floor. Create a new 3D view and follow the steps outline above.
5. After you have all the floors setup in 3D with cropped boundaries, you can export them to AutoCAD as individual DWGs. (File menu > Export > CAD Formats)
Autodesk has released a white paper on this with more information: