This bug has been logged by ESRI, search for the ID NIM071017.
A work around we came up with was to create a temp dataset and create CAD formatted feature classes there. Once all the feature classes we need are in the temp dataset, we would set the workspace to that dataset, get a list of the feature classes, and use that list as the first input to the ExportCAD tool. Its still not ideal and I hate creating and saving intermediate data but it works. Hopefully that helps.