You might try posting this question in an AutoCAD forum as well. I know that Map 3D has the ability to connect to an ArcGIS database and I *think* a simple connection is a fairly straightforward approach using a database connection. The difficulty is that AutoCAD is engineering software and ArcGIS is mapping software and they just don't play nice. I had the same problem where Map 3d users needed to connect to SDE but we did not want them to be able to edit the data - just add it to their drawings. Our solution was to create a web feature service using OpenGeo. It's an open source GIS suite that was downloaded and installed on our webserver. In our solution it basically serves as the "middleware" between ArcSDE and Map3D to create an OGC compliant web service, which AutoCAD can consume easily. Once it was up and running we had to create tableviews in SDE and created the web feature service from them. I know this isn't the answer you were looking for, but maybe it'll provide a little insight~