An update on this, which for me at least is very relevant. I just got a phone call from a happy CAD user, which must be a tautology.....
To export dxf from QGIS (and get said happy CAD user) use the plugin MMQGIS. The CAD users grumble a lot if you export a fully formed CSV by the layer saveas option in QGIS (note this thread Getting list of coordinates for points in layer using QGIS?). The saveas dxf option in 1.7.4-3 and trunk was not satisfactory either, despite the reported improvements in trunk.
How to spread the joy;
Have your shapefile in the projection of UTM and a modern map gid for your region. Export it to this if you must, don't expect OTFR to do the heavy lifting for you.
Fetch the mmqgis from the plugin manager, you may find that mmqgis does not fall aphabetically where it should, last time I looked it had made it to the QGIS contributed repo. And you may find that mmqgis is turned off by default, manage the plugin and turn it on.
Operate the plugin and choose the Geometry Export to CSV File, for a polyline it creates two files, you need to logical with your names and let it use the attributes suffix for the.... attributes. This increases the joy of the end user when they can easily interpret what they have been given.
By all accounts the import to AutoCAD is a breeze, with no of that tedious "parsing" of columns or descent into manual manipulation of data.
So many thanks to all the QGIS team, the mmqgis plugin and GIS.SE for allowing me to write my very own help file for when I forget how to do it!!
As far as importing from AutoCAD, I am still waiting for the joyous operator to pay a bit more attention and try to resolve the same issue which is noted in this thread by another GIS trooper - Last edited information missing when used QGIS - Dxf to Shp