I believe the DWG launcher should be the primary association. It gets updated for each release and keeps track of all valid autodesk products on your machine. So if you run AutoCAD 2009 and then close it and then double-click a DWG file, it should open it with AutoCAD 2009. Then if you use Civil 3D 2011, the next time you open a DWG file it will use Civil 3D 2011. It should probably prioritize AutoCAD over TrueView even if TrueView was used last but I'm not sure on that.
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