Hi Jim,
I assume that Civil 3D has the same data provider limitations as Map 3D, so I don't think that you can connect to the v10 of ArcSDE. Take a look at this post for more info:
http://forums.autodesk.com/t5/AutoCAD-Map-3D/AutoCAD-Map-3D-and-ArcGIS-10/td-p/2829619
Also, there are more general problems when connecting to any ESRI product by FDO if you are on a 64 bit system. ESRI does not really support 64 bit, so you may not be able to connect at all if you are working from a 64 bit machine. OS GEO has some 64 bit providers available (http://fdo.osgeo.org/content/fdo-360-downloads), but I have not heard from anyone as to whether or not they are working.
Norm