Jim Light sent this useful tip for GIS users who develop their shape files in AutoCAD. The tip allows the user to export the files in 3D.

First select the mapexport command, then select the line option. Next, expand the data tab to select attributes from either object data and/or object properties. Finally, select options and then driver options where you can select three-dimensional. This is sort of a hidden location in the command.   

Notes from Cadalyst tip reviewer R.K. McSwain: This is an excellent tip regarding a more or less hidden feature in AutoCAD Map. A lesson to take from this is to explore commands fully, especially commands with dialog boxes. You may just find something to help you. Jim tested this in AutoCAD Map 2013 and note that AutoCAD Civil 3D contains the same option in the mapexport command.