Writing a Python script for converting geodatabase layers, personal or SDE, to CAD produces empty output layers.
This is a known issue at ArcGIS 9.0
The ArcGIS Desktop Help provides examples of the syntax that should be used to write this code. The examples provided for this command contain typos. For example, the output type could be DXF_R14, however in the help, this is listed as DXF-R14, using a hyphen instead of an underscore.